Conclusion of marriage under some circumstances is prohibited by law.
Marriage conclusion is prohibited:
a) if at least one of the persons is in a marriage, registered by the procedure established by the law;
b) between close relatives (direct relatives: parents and children, grandparents and grandchildren, as well as siblings and siblings with common mother or common father, aunts, uncles and cousins);
c) between the adopters and adopted;
d) between the persons, at least one of who is recognized incapable by court.

Source: Iravaban.net