I'm going to make a legal argument for incestuous polygamy. Bear with me.
Let's say you live with your sister and her two children from a previous marriage. They live in your home, depend on your income and so on. You would like to deduct your sister and her kids as dependents on your taxes. Your sister would like you to have custody of the kids instead of her ex in case anything happens to her. So you decide to marry your sister, legally, even though you are already happily married.
Why shouldn't you and your sister be allowed to "marry" in the sense that you could both sign a legal document that grants you all the legal benefits of marriage? You could even stipulate that your wife agrees to join the marriage, too. Why couldn't you do the same thing for your brother?
There's nothing amoral about it.