I'm going to send a confrontational email to my father-in-law for encouraging his daughters to interact with their brother after he sexually abused them
Oh my goodness, it is terrible that their brother has done that.
I know it is. I've put out a restraining order order on him.
Instead of sending an email, you should tell the authorities about it. That cannot go unpunished.
Nope. It's all in my family's past.
Even if it is in the past, there is no reason to think that he may not do it again in the future. The sisters are in danger whenever he is around, he needs to be locked up.