Leviticus 20:17, 19

If a man marries his sister,  whether his father’s daughter or his mother’s daughter, and they have sexual relations,  , it is a disgrace.  They are to be cut off publicly from their people. He has had sexual intercourse with his sister; he will bear his iniquity.

You must not have sexual intercourse with your mother’s sister or your father’s sister,  for it is exposing one’s own blood relative; both people will bear their iniquity.