Affinity in Law: Degrees, Recusal, and Marriage Limits

Affinity in law is the family relationship created through marriage between you and your spouse’s blood relatives. Your mother-in-law, your stepchildren, your brother-in-law, your spouse’s grandparents: all are your relatives by affinity. The distinction between affinity and consanguinity (blood relation) is not academic. It decides when a federal judge must step aside from a case, who a federal official cannot hire, who you may claim as a tax dependent, whether a stepchild can immigrate, and whether two family-owned businesses count as one for small business contracting.

Affinity vs. Consanguinity

Consanguinity links people who share a biological ancestor. Affinity links people who became family through someone’s marriage. When you marry, every blood relative of your spouse becomes your relative by affinity, and your spouse becomes a blood-equivalent for counting purposes under most statutes that reach in-laws.

One boundary that catches people: affinity runs through the marriage, not around it. Your brother and your spouse’s sister are not legally related to each other. They sit at the same holiday table, but the law sees two unrelated people. Affinity connects you to your spouse’s relatives. It does not connect your relatives to your spouse’s relatives.

Degrees of Affinity

Statutes and courts measure how close an affinity relationship is by counting degrees. Each step between your spouse and their relative becomes one degree of affinity between you and that relative.

First degree: your spouse’s parents and your spouse’s children from another relationship.
Second degree: your spouse’s siblings, grandparents, and grandchildren.
Third degree: your spouse’s aunts, uncles, nieces, nephews, and great-grandparents.

Federal law counts these degrees under the civil law method, which follows the chain of relationship up to a common ancestor and back down. The numbers matter because statutes use them to draw bright lines, often cutting off at the second or third degree.

Judicial Recusal

The highest-profile use of affinity is judicial disqualification. Under 28 U.S.C. § 455, a federal judge must step aside from any case in which 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. The statute specifies the civil law method for calculating degrees.¹

In practice, a federal judge whose brother-in-law represents one of the parties cannot hear the case. Neither can a judge whose spouse’s niece is a named plaintiff. Failing to recuse can get a judgment reversed on appeal.

The same logic reaches juries. During jury selection, a potential juror related by marriage to any participant in the trial can be removed for cause, and challenges for cause carry no numerical cap.

Anti-Nepotism in Federal Hiring

Federal anti-nepotism law targets affinity directly. Under 5 U.S.C. § 3110, a public official cannot appoint, employ, promote, or advocate for the hiring of a relative into a civilian position in the agency the official controls. The statute defines “relative” to include a long list of in-laws and step-relatives: father-in-law, mother-in-law, son-in-law, daughter-in-law, brother-in-law, sister-in-law, stepfather, stepmother, stepson, stepdaughter, stepbrother, and stepsister.²

The penalty cuts off the paycheck. A person hired in violation of the statute is not entitled to pay, and no Treasury money may be used to compensate them.² State anti-nepotism rules vary; many restrict hiring within the second or third degree of affinity and some add fines or removal from office.

Taxes

Affinity shows up at tax time in a specific way. Under 26 U.S.C. § 152, the IRS treats several in-law relationships as qualifying for dependent status. A father-in-law, mother-in-law, brother-in-law, sister-in-law, son-in-law, or daughter-in-law may be claimed as a qualifying relative if they live with you or get more than half their support from you, and if they meet the gross-income limit and are not someone else’s qualifying child.³

Gift tax rules ignore affinity. The 2026 annual exclusion of $19,000 per recipient applies the same way whether the recipient is your in-law, a blood relative, or a stranger. Only your spouse gets different treatment, through the unlimited marital deduction.⁴

Intestate succession is a hard limit. If someone dies without a will, in-laws inherit nothing in any state. A son-in-law or mother-in-law takes from your estate only if your will says so.

Immigration and Social Security

Affinity drives large consequences in immigration, though the Immigration and Nationality Act uses the word “stepchild.” Under INA § 101(b)(1)(B), a stepchild qualifies as a “child” for immigration purposes only if the marriage creating the step-relationship took place before the child turned 18.⁵ Miss that date by a day and the stepchild cannot be a principal beneficiary on an immigrant visa petition filed by the stepparent.⁶

A stepchild can keep immigration eligibility after a divorce or the death of the natural parent, provided the actual relationship with the stepparent continues. The State Department looks for ongoing contact: calls, letters, emails.⁶

Social Security has its own timing rules. A stepchild claiming survivors benefits must have held that status for at least nine months before the stepparent’s death. For disability or retirement benefits during the stepparent’s life, the stepchild status must have existed for at least one year before filing. Accidental or line-of-duty deaths carry exceptions.⁷

Small Business Affiliation

Family relationships can combine two businesses into one for SBA size purposes. Under 13 C.F.R. § 121.103, firms owned or controlled by married couples, parents and children, or siblings are presumed affiliated if they do business together through subcontracts, joint ventures, shared loans, equipment, locations, or employees. When affiliation applies, the SBA adds the firms’ revenues and employees together, which can push a company past the size standard for a set-aside contract.⁸

Worth noting here, because the circle is narrower than many expect: the family-based presumption reaches married couples, civil union partners, parents, children, and siblings only. In-laws beyond those categories do not trigger automatic affiliation.⁸ The presumption can also be rebutted by showing a clear line of fracture between the companies in management, finances, and resources.

Attorney Conflicts

Lawyers face their own version of the recusal problem. Under ABA Model Rule 1.7, a lawyer has a personal-interest conflict when opposing counsel is a spouse or close relative by marriage. Comment 11 points to the risk that confidential information will leak at the family table and that loyalty to the client will yield to family harmony.⁹

Both lawyers must disclose the relationship and obtain informed consent from their clients. Without consent, they should withdraw. The conflict is personal to the individual lawyer, so it generally does not spread to the rest of the firm.⁹

Marriage, Incest, and Notaries

Many states prohibit marriage between certain affinity relatives, most commonly between a stepparent and stepchild. The scope varies. Some states bar lineal affinity marriages even after the original marriage ends; others allow them once the spouse who created the connection has died. Check your state’s family code before applying for a license.

Some state incest statutes also reach affinity. Michigan covers people related by blood or affinity to the third degree. Colorado’s statute covers stepchildren who are 21 or older. Sentencing for an incest conviction ranges widely by state and by the specific relationship.

Notary rules in many states prohibit notarizing documents for close relatives, including in-laws, with the exact reach varying from no limit at all to coverage of step-relatives and domestic partners. In community property states, notarizing for a spouse is often treated as a conflict because of the notary’s own stake in community assets. Witness competency is the opposite story: a relative by marriage can testify in civil or criminal proceedings, and the opposing side’s remedy is cross-examination, not exclusion.

When Affinity Ends

Divorce generally severs affinity. Once a final decree issues, former in-laws become legal strangers for most purposes, including judicial recusal and nepotism. A judge who once had to step aside because of a brother-in-law may no longer need to.

Children are the main exception. When a marriage produced children, some states treat the affinity link as continuing for specific purposes, especially in nepotism and conflict-of-interest contexts, because the family lines remain connected through the children. Immigration law takes the same approach: stepchild status survives divorce as long as the actual relationship does.

Death of a spouse is less settled. Some jurisdictions treat affinity as ending at death; others preserve it for limited purposes like marriage prohibitions. A handful of states still bar a widower from marrying his deceased wife’s daughter even though the marriage that created the relationship no longer exists.

1
Office of the Law Revision Counsel. 28 USC 455 – Disqualification of Justice, Judge, or Magistrate Judge
2
Office of the Law Revision Counsel. 5 USC 3110 – Employment of Relatives; Restrictions
3
Office of the Law Revision Counsel. 26 USC 152 – Dependent Defined
4
Internal Revenue Service. Frequently Asked Questions on Gift Taxes
5
U.S. Citizenship and Immigration Services. Volume 6 – Immigrants, Part B – Family-Based Immigrants, Chapter 2 – General Eligibility Requirements
6
U.S. Department of State. 9 FAM 102.8 Family-Based Relationships
7
Social Security Administration. Stepchild-Stepparent Relationship
8
eCFR. 13 CFR 121.103 – How Does SBA Determine Affiliation?
9
American Bar Association. Rule 1.7 Conflict of Interest Current Clients – Comment