If you and your stepbrother wish to marry, you are free to do so. No U.S. state makes it illegal for stepsiblings to marry because stepsiblings are not related by blood; thus, they are not affected by consanguinity laws. Consanguinity laws make it a crime for close blood relatives to marry, such as a parent and child, two siblings, an aunt or uncle and niece or nephew, or a grandparent and grandchild. Only 19 states allow first cousins to marry, and 25 states prohibit marriages between first cousins. All states allow second cousins to marry, but no state has a law that prevents stepsiblings from marrying. Half-siblings are related by blood and cannot marry.
Consanguinity is a close blood relationship between people who have descended from the same persons. Linear consanguinity refers to a blood relationship that exists in a straight line, such as parent and child or grandparent and grandchild, and so on up or down the ancestral or descendant line. All 50 states prohibit marital unions between relatives that share linear consanguinity.
Stepsiblings are not related by blood, although they do have an affinity with each other – what people commonly call, “related by marriage” in that a stepbrother's mother may have married his stepsister's father, or in a similar combination. In legal terms, “related by affinity" doesn't refer to a blood relationship or shared genetics. Sometimes, a stepbrother and stepsister will fall in love with each other and the two may wish to marry. Since stepsiblings are not blood relatives, they are legally free to do so. No state law prohibits this union, and stepsiblings are not at risk of producing children with genetic defects due to consanguinity.
Marrying Your Cousin
Sometimes, first cousins fall in love and wish to marry. Unlike stepsiblings, first cousins are closely-related by blood. In fact, 25 states prohibit marriage of first cousins -- including Arkansas, Minnesota, New Hampshire, Pennsylvania, Texas, Washington and Wyoming -- because of consanguinity. In five states -- Arizona, Illinois, Maine, Utah and Wisconsin -- marriage between first cousins is only allowed if one or both are older (generally, one or both must be older than 50 or 55, but sometimes over 65) and one is unable to reproduce. In Maine, the couple must present a doctor's certificate that they have undergone genetic counseling.
Marriage between first cousins is legal in 19 states, which includes Alaska, California, Colorado, Connecticut, Georgia, Hawaii, Massachusetts, New York, and the District of Columbia. Marrying a second cousin is allowed in all 50 states. A second cousin is your first cousin’s child, who is a second cousin to your child.
Even though no law prohibits stepsiblings from marrying, your parents, close relatives or step-relatives may not be comfortable with this. Reassure them that no blood relationship exists -- and that it would be impossible to bear children with genetic defects that result from consanguinity since no shared genetics exist, and any genetic defects in children would be because of other factors.