Degrees of Consanguinity and Kinship in Law: How to Count and Apply

Degrees of consanguinity measure how many generational steps separate two blood relatives, counted through their nearest common ancestor. Under the civil law method used by federal statutes and most state courts, a parent and child are first-degree relatives, siblings are second-degree, an aunt or uncle is third-degree, and first cousins are fourth-degree. The lower the number, the closer the legal relationship, and the stronger the rights and restrictions that come with it in inheritance, marriage, immigration, tax, medical decision-making, and judicial ethics.

How to Count a Degree of Relationship

The civil law method works as a two-part count. Start at one person, move up one generation at a time until you reach the ancestor both people share, then move back down to the other person. Add the two legs together.¹

For people in a direct line, the count is just the generation gap. Parent to child is one step, so they are first-degree relatives. Grandparent to grandchild is two steps, so second-degree.²

Collateral relatives branch off sideways from a shared ancestor, and the count runs up one line and back down the other. Siblings: one step up to the shared parent, one step down, for a second-degree relationship. Aunt or uncle to niece or nephew: two steps up to the grandparent, one step down, for third degree. First cousins: two steps up to the shared grandparent, two steps back down, for fourth degree.² That last one surprises people who think of cousins as close family. By the math, a first cousin is the same distance from you as a great-grandparent.

Watch Out for the Common Law Method

A rival counting system, inherited from English common law, measures only the longer of the two lines to the common ancestor. Under that method first cousins come out as second-degree relatives rather than fourth. Federal law and most modern state codes use the civil law method, but some older statutes still reference the common law count. When a statute talks about “degrees of relationship,” check which system it names before you do the arithmetic. The same two people can carry different numbers depending on which rulebook you open.

Consanguinity, Affinity, and Adoption

Consanguinity is strictly a blood tie. Affinity is the legal bond created by marriage, connecting you to your spouse’s blood relatives. Your mother-in-law is a first-degree relative by affinity, not by consanguinity. Both count for many legal purposes, but statutes often treat them separately, so a rule that reaches “relatives within the third degree of consanguinity” does not automatically cover in-laws unless the statute says so. Some rules, such as the federal anti-nepotism statute, expressly include both.³

Adoption creates a full legal parent-child relationship, giving the adopted child the same status and inheritance rights as a biological child. It also cuts the legal tie to the biological parents in most cases, ending inheritance rights in that direction. Stepparent adoptions are the main exception: the child keeps the legal relationship with the biological parent married to the adopting stepparent.

Most states also recognize equitable adoption, which protects a child raised under an informal promise to adopt that was never legally completed. It generally requires an express or implied agreement to adopt, surrender of the child to the would-be adoptive parent, and enough time living as part of the family that denying inheritance would be a fraud on the child.⁴ Claims usually surface in probate, when someone never formally adopted tries to inherit from a deceased caregiver.

Inheritance When There Is No Will

When someone dies intestate, state law distributes the estate by kinship degree. Most states follow the Uniform Probate Code pattern, ranking heirs in a strict tier order: surviving spouse and children first, then parents, then siblings, then grandparents, then outward.⁵ The estate reaches a lower tier only when nobody in a higher tier survives. If no relative can be found at any level, the estate escheats to the state.

Within a tier, another rule decides what happens when an heir has predeceased the decedent. Per stirpes distribution (“by branch”) sends the deceased heir’s share to that heir’s own descendants, keeping it in the family line. Per capita distribution (“by head”) divides the estate only among living members of the class, giving nothing to a deceased heir’s branch. Wills and trusts usually pick one method; when they are silent, state law fills in.

Marriage Bans by Degree

Every state prohibits marriage between close blood relatives, though the exact cutoff varies. First-degree relatives (parent and child) and second-degree relatives (siblings, including half-siblings) are barred everywhere, and those marriages are void from the start. Nearly all states also prohibit grandparent-grandchild marriages and aunt or uncle unions with a niece or nephew.

First cousin marriage is the fault line. Roughly 19 states permit it, about 24 prohibit it outright, and the rest allow it only under conditions such as genetic counseling or a minimum age. Some states extend the bar to affinity relationships, prohibiting a marriage between a stepparent and stepchild even after the original marriage ends.

Sexual conduct between close relatives is separately criminalized in most states, whether or not the parties tried to marry. Penalties range from lower-level felonies with a few years in prison to sentences of 10 to 20 years, and a handful of states require sex offender registration on conviction. Two states do not criminalize consensual incest between adults, but both still refuse to recognize incestuous marriages.

Family-Based Immigration Tiers

U.S. immigration law sorts sponsorship eligibility entirely by kinship. The most favorable category, “immediate relatives,” is limited to the spouse, unmarried minor child, or parent of a U.S. citizen, and the citizen must be at least 21 to petition for a parent.⁶ Immediate relatives are not subject to annual numerical caps, so waits are shorter than in other family categories.⁷

More distant relationships fall into four capped preference categories with backlogs measured in years or decades:

First preference (F1): unmarried adult sons and daughters of U.S. citizens.
Second preference: spouses and children of lawful permanent residents (F2A), and unmarried adult sons and daughters of permanent residents (F2B).
Third preference (F3): married sons and daughters of U.S. citizens.
Fourth preference (F4): brothers and sisters of U.S. citizens, if the citizen is at least 21.

⁸

The F4 sibling category, a second-degree blood relationship, often has the longest waits of all. Derivative benefits do not attach to the immediate relative category, so the spouse of a minor child, for example, must qualify on their own.⁷ Kinship also governs financial sponsorship. If a petitioner dies, a substitute sponsor on the Affidavit of Support (Form I-864) must be a close relative of the immigrant, such as a spouse, parent, sibling, or adult child.⁹

Medical Decisions and Organ Donation

If you cannot speak for yourself and never named a healthcare agent, state law hands the decision to your closest available relative. Most states use a priority ladder that begins with a spouse or domestic partner, then an adult child, then a parent, then a sibling, and in some states a close friend. When several people share the same rung, such as multiple adult children, some states allow a majority to decide and others require the group to designate one spokesperson.

The Revised Uniform Anatomical Gift Act, adopted by most states, uses a similar hierarchy for authorizing an anatomical gift when the deceased left no instructions: healthcare agent, spouse, adult children, parents, adult siblings, adult grandchildren, grandparents, and finally an adult who showed special care for the deceased.¹⁰ A person higher on the list can override an objection from someone lower.

Medical records follow a related but separate rule. Under HIPAA, a personal representative authorized by state law to make your healthcare decisions has the same access to your protected health information that you would have. For a deceased person, that role goes to the executor or administrator. Providers may also share information with family members involved in your care as long as you have not objected, and if you are incapacitated they can disclose information when professional judgment says it serves your best interest.¹¹

Tax Rules That Turn on Kinship

The biggest tax break tied to family status belongs to spouses. The unlimited marital deduction lets one spouse transfer any amount of property to the other at death without federal estate tax.¹² Transfers to anyone else are subject to federal estate tax once the estate exceeds the basic exclusion, which is $15,000,000 per individual for 2026.¹³ Married couples can effectively double that through portability of the deceased spouse’s unused exclusion.

During life, you can give up to $19,000 per recipient per year without filing a gift tax return or dipping into your lifetime exclusion.¹³ The exclusion is per giver, so a married couple can give $38,000 a year to each child or grandchild tax-free. The annual exclusion is adjusted periodically for inflation in $1,000 increments.¹⁴

Family status also brings restrictions. The IRS defines “related persons” to include your spouse, siblings, half-siblings, ancestors such as parents and grandparents, and lineal descendants such as children and grandchildren.¹⁵ Transactions between related persons draw extra scrutiny. You generally cannot deduct a loss on a sale to a family member, and below-market loans to relatives can trigger imputed interest income.

Judicial Recusal and Anti-Nepotism

Federal judges must step aside from any case where a person within the third degree of relationship to the judge or the judge’s spouse is a party, is acting as a lawyer, has a financial interest that could be substantially affected, or is likely to be a material witness. Under the civil law count the statute uses, third-degree relatives include parents, children, siblings, grandparents, grandchildren, aunts, uncles, nieces, and nephews.¹ The same kinship logic drives strikes for cause during jury selection.

The federal anti-nepotism statute reaches further. A public official cannot appoint, promote, or advocate for the advancement of a relative within the official’s own agency. “Relative” for this purpose covers parents, children, siblings, uncles, aunts, first cousins, nephews, nieces, spouses, and various in-laws and step-relatives, out to the fourth degree of consanguinity and including affinity ties.³ A violation can void the appointment and expose the official to administrative consequences, though the hired relative bears no personal liability.

1
Office of the Law Revision Counsel. 28 USC 455 – Disqualification of Justice, Judge, or Magistrate Judge
2
Consiglio Nazionale del Notariato. Blood Relations and Relations by Marriage
3
Office of the Law Revision Counsel. 5 USC 3110 – Employment of Relatives; Restrictions
4
Social Security Administration. Equitable Adoption – General
5
University of Michigan Law School Scholarship Repository. Intestate Succession Under the Uniform Probate Code
6
Office of the Law Revision Counsel. 8 USC 1151 – Worldwide Level of Immigration
7
U.S. Citizenship and Immigration Services. Green Card for Immediate Relatives of U.S. Citizen
8
U.S. Citizenship and Immigration Services. Green Card for Family Preference Immigrants
9
U.S. Citizenship and Immigration Services. Form I-864 Instructions for Affidavit of Support Under Section 213A of the INA
10
Health Resources and Services Administration. Ethics of Deceased Organ Donor Recovery
11
U.S. Department of Health and Human Services. Under HIPAA, When Can a Family Member of an Individual Access the Individual’s PHI from a Health Care Provider or Health Plan?
12
Office of the Law Revision Counsel. 26 USC 2056 – Bequests, Etc., to Surviving Spouse
13
Internal Revenue Service. What’s New – Estate and Gift Tax
14
Office of the Law Revision Counsel. 26 USC 2503 – Taxable Gifts
15
Internal Revenue Service. Related Person