A blood relationship, called consanguinity in legal terms, is the biological connection between two people who descend from a common ancestor, and the law measures it in numbered degrees by counting the generational steps between them. That number isn’t academic. It decides who inherits your property if you die without a will, whom you can legally marry, how much a state charges your heirs in inheritance tax, which relatives you can sponsor for a green card, and which family members can collect Social Security after you die.
How Degrees Are Counted
American courts use the civil law method. You count every generational step from one person up to the nearest shared ancestor, then back down to the other person, and add them together. A parent and child are separated by one step, so they share a first-degree relationship. Siblings are each one step from a shared parent, which makes them second-degree relatives. First cousins are each two steps from a shared grandparent, placing them at the fourth degree.
Relationships also split into two shapes. Lineal consanguinity runs in a straight vertical line: parent to child, grandparent to grandchild, each generation adding one degree. Collateral consanguinity covers relatives who share an ancestor without descending from each other, like siblings, aunts and uncles, or cousins. Courts use a standardized Table of Consanguinity to slot every family tie into a numbered degree.
Who Counts as a First-, Second-, or Third-Degree Relative
- First degree: parents and children.
- Second degree: grandparents, grandchildren, and siblings.
- Third degree: great-grandparents, great-grandchildren, aunts, uncles, nieces, and nephews.
Half-blood relatives, who share only one biological parent, sit at the same degree as whole-blood relatives on the chart, but states don’t always treat them identically for inheritance. More on that below.
Inheritance When There’s No Will
Intestacy is the clearest place a degree of relationship controls an outcome. When someone dies without a valid will, state law distributes the estate according to a priority ladder, and most states follow the structure of the Uniform Probate Code.1Legal Information Institute. Uniform Probate Code The general order:
- A surviving spouse typically takes the entire estate when the deceased left no children or parents. When children or parents survive, the spouse’s share shrinks but usually remains the largest.
- Biological and legally adopted children split the remainder equally after the spouse’s share. If a child predeceased the parent, that child’s own children may step into the share.
- Parents inherit if no spouse or descendants survive.
- Siblings, and then their children, inherit when no spouse, descendants, or parents survive.
- Grandparents, aunts, uncles, and cousins inherit only when no closer relative can be found.
On half-blood relatives, states diverge. The Uniform Probate Code treats half-blood and whole-blood relatives identically, and most states that have adopted the code follow suit. A handful of states still follow older common law and give a half-blood sibling only half the share of a whole-blood sibling when both compete for the same inheritance.
Who Pays Inheritance Tax Based on Relationship
Only six states impose an inheritance tax: Iowa (effectively phased out), Kentucky, Maryland, Nebraska, New Jersey, and Pennsylvania. In those states, your rate depends directly on how closely you were related to the person who died.
The pattern holds across all six. Spouses are always exempt. Children, grandchildren, and parents pay nothing or very little. Siblings face moderate rates. Nieces, nephews, aunts, and uncles pay more, and anyone outside the family tree pays the top rate. In Pennsylvania, children and grandchildren pay 4.5%, siblings pay 12%, and everyone else pays 15%. Kentucky exempts close family but charges 4% to 16% for nieces, nephews, and in-laws, with higher effective rates for unrelated beneficiaries.
These rates sit on top of any federal estate tax. For 2026, the federal estate and gift tax basic exclusion is $15,000,000 per person, so only estates above that threshold owe federal estate tax.2Internal Revenue Service. What’s New – Estate and Gift Tax Maryland is the only state that imposes both a state estate tax and an inheritance tax.
Marriages Blood Relatives Can’t Enter
Every state prohibits marriage between close blood relatives. The ban universally covers lineal relatives (parent and child, grandparent and grandchild) and siblings, including half-siblings. Most states extend it to aunts and nephews, and uncles and nieces.
A marriage that crosses those lines is void ab initio, meaning it was never legally valid from the moment it was performed.3Legal Information Institute. Void Marriage That’s different from a voidable marriage, which stays valid until a court sets it aside. A void incestuous marriage doesn’t require a divorce. Either party, or a state official, can seek annulment at any time, and the law treats the marriage as though it never existed. That has real consequences for property division, inheritance rights, and benefit eligibility.
Cousin marriage is the one place state rules fracture. About 16 states and the District of Columbia allow first-cousin marriage without restriction. More than 30 states ban it. A few states permit it only under specific conditions, such as genetic counseling, proof of infertility, or a minimum age.
Most states also treat incest as a criminal offense separate from the marriage ban, often as a felony. Sexual relationships between close blood relatives outside marriage can draw prosecution on their own.
How Immigration Sponsorship Depends on Relationship
U.S. immigration law sorts family-based petitions by how closely the sponsor is related to the beneficiary, and that placement determines whether the wait is months or decades.
Immediate relatives of U.S. citizens, defined as spouses, unmarried children under 21, and parents (if the citizen is at least 21), get the fastest path. Visas for immediate relatives are unlimited, with no annual cap and no backlog.4U.S. Citizenship and Immigration Services. Green Card for Immediate Relatives of U.S. Citizen
Everyone else falls into one of four family preference categories, each subject to annual numerical limits:5U.S. Citizenship and Immigration Services. Green Card for Family Preference Immigrants
- First preference (F1): unmarried sons and daughters, 21 or older, of U.S. citizens.
- Second preference: spouses and children of lawful permanent residents (F2A), and unmarried sons and daughters 21 or older 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, as long as the citizen is at least 21.
The further out on the family tree, the longer the wait. F4 petitions for siblings from high-demand countries can take more than 20 years.
How the IRS Defines “Family” for Related-Party Rules
The IRS uses a narrower definition of family than most people assume, and the degree of relationship matters for one rule in particular. Under IRC Section 267, if you sell property at a loss to a family member, you cannot deduct the loss. For this purpose, “family” means your spouse, siblings (including half-siblings), ancestors (parents, grandparents), and lineal descendants (children, grandchildren).6Office of the Law Revision Counsel. 26 U.S. Code 267 – Losses, Expenses, and Interest With Respect to Transactions Between Related Taxpayers Cousins, aunts, uncles, nieces, and nephews are not covered. The rule exists to stop taxpayers from manufacturing deductible losses by shuffling property among close relatives.
Gift tax doesn’t work this way. The 2026 annual gift exclusion is $19,000 per recipient and applies the same whether the recipient is a relative or a stranger.7Internal Revenue Service. Frequently Asked Questions on Gift Taxes Gifts above that count against the $15,000,000 lifetime exclusion.
Who Qualifies for Social Security Survivors Benefits
When a worker who paid into Social Security dies, certain family members can collect monthly survivors benefits based on the worker’s earnings record. The system is built around the nuclear family, so your relationship either qualifies you or it doesn’t:8Social Security Administration. Who Can Get Survivor Benefits
- A surviving spouse is eligible at age 60, or age 50 with a disability, if married at least nine months before the death and not remarried before age 60. A surviving spouse caring for the deceased’s child under 16 can collect at any age.
- A surviving divorced spouse qualifies under the same age rules if the marriage lasted at least 10 years.
- Unmarried children under 18 qualify, as do children up to 19 who are still in elementary or secondary school full time. A child with a disability that began before age 22 can collect at any age.
- Dependent parents aged 62 or older qualify if they received at least half their support from the deceased worker.
A one-time lump-sum death payment of $255 is also available to a surviving spouse or qualifying child, if applied for within two years of the death.9Social Security Administration. Survivors Benefits Siblings, cousins, aunts, and uncles do not qualify for Social Security survivors benefits regardless of how close the relationship was in life.
How to Prove a Blood Relationship
Any of the rights above depend on documenting the relationship to a court’s or an agency’s satisfaction.
DNA testing is the strongest proof. For results to hold up in court or satisfy a government agency, the test must come from a laboratory accredited under the standards published by AABB (formerly the American Association of Blood Banks, now the Association for the Advancement of Blood & Biotherapies).10AABB. Standards for Relationship Testing Laboratories The 17th edition of those standards took effect January 1, 2026. An at-home consumer DNA kit won’t satisfy a court because there’s no verified chain of custody.
A certified birth certificate listing both parents remains the baseline document for proving parentage. USCIS requires primary evidence like a birth certificate to establish a qualifying relationship for immigration petitions. When a birth certificate isn’t available or isn’t reliable, the agency considers secondary evidence like church records, school records, or census documents, and failing those, sworn affidavits from people with firsthand knowledge.11U.S. Citizenship and Immigration Services. Volume 6 – Immigrants, Part B – Family-Based Immigrants, Chapter 4 – Documentation and Evidence
Unmarried parents can establish legal parentage without going to court by signing a voluntary acknowledgment of paternity. Federal regulations require states to make these forms available at hospitals, birth record agencies, public health clinics, and child support enforcement offices.12eCFR. 45 CFR 303.5 – Establishment of Paternity Both parents must sign, and signatures must be authenticated by a notary or witness. The acknowledgment carries the legal weight of a court judgment, and challenges are typically barred after two years.
Married parents don’t need any of that for the father. In virtually every state, if a couple is married when a child is born, the husband is automatically presumed to be the legal father. The presumption is rebuttable through genetic testing or a legal proceeding, but states often impose strict time limits, sometimes as short as two years after birth, for bringing that challenge.
When voluntary acknowledgment isn’t available and the marital presumption doesn’t apply, a court proceeding to adjudicate parentage is the remaining route. Under the Uniform Parentage Act, which many states have adopted in some form, a court will declare someone a parent if genetic testing identifies them as the biological parent and that identification isn’t successfully challenged, if they admit parentage at a hearing, or if they were served and failed to appear. Once the court issues its order, the legal parent gains rights like custody and visitation and takes on obligations, most notably child support.