Marriages between ancestors and descendants of any degree, of a stepfather with a stepdaughter, stepmother with stepson, between uncles and nieces, aunts and nephews, except in cases where such relationship is only by marriage, between brothers and sisters of the half as well as the whole blood, and first cousins are declared to be incestuous, illegal and void, and are expressly prohibited. Provided, that any marriage of first cousins performed in another state authorizing such marriages, which is otherwise legal, is hereby recognized as valid and binding in this state as of the date of such marriage.