Fraternal marriage is a type of polygamous marriage.
In this type of marriage, a woman marries more than one man.
There are two types of polygamy.
$(1)$ Fraternal polygamy:
In marriages in which the men who join the woman are relatives, it is called fraternal polygamy.
$(2)$ Intrauterine polygamy:
If there is no relationship between men who marry a woman and they are not relatives, then such a relationship is called polygamy.
In short, when a woman marries more than one man and these men are relatives, the marriage is called a fraternal marriage.