But there is definitely good biology behind the laws that prohibit brothers and sisters from having children. The risk for passing down a genetic disease is much higher for siblings than first cousins.

Can a brother and sister get married?

Certain blood relatives may not legally marry each other. This includes marriages between siblings (sibling means a brother, sister, half-brother or half-sister) and between a parent and child (for example; a mother and son or father and daughter).

Is it legal to have a baby with your brother?

In the United States, incest laws prohibited intimate relationships between children and parents, siblings, and grandchildren and grandparents. Almost all states criminalize incest between adults. Some states also prohibit intimate relations between aunts, uncles, nieces, nephews, and cousins.