I have a question about the topic of incest. Is it a sin, or forbidden for a half sibling to marry another half sibling (same mothers)? If so, where is the scriptural basis for it? If not, is it against the American law?
I believe it is against American law, but a lawyer would be better able to answer that part of the question. Generally it is considered incest to have sex with any blood relative (from either parent) closer than a cousin. There is certainly scripture against it. The Jews were told in the Law of Moses that it was "a wicked thing." "And if a man shall take his sister, his father's daughter, or his mother's daughter, and see her nakedness, and she see his nakedness; it is a wicked thing; and they shall be cut off in the sight of their people: he hath uncovered his sister's nakedness; he shall bear his iniquity." (Leviticus 20:17)