My son in law is a flaming lib. He has turned my daughter as well.
IMHO, the only solution to your situation is to refuse (militantly if necessary) to talk about politics with your son in law. You are in an otherwise impossible situation unless you want to risk all (your daughter) and go to war.
Thats ok. I Got them the other day when they told me they were Expecting. I stone cold asked them if they had decided to keep it or abort it. You should have seen their faces.