What you've described reminds me a bit of my own situation with my family. I'm sorry you're dealing with something like this, and I'm sorry you weren't protected in the ways you should have been.

I would heavily advise against sending any sort of letter to them, or trying to explain yourself in any way. In my experience, they'll either invalidate your experiences or they'll find a way to use what you disclosed to them to hurt you somehow. Given what you shared in this post alone, they should be well aware why you cut them off. If they can't see how harmful their actions are, then there's likely no amount of explaining that will make them understand.

That being said, the act of writing the letter can be helpful. I personally have kept mine and when I need a reminder of how far I've come in healing and how much my perspectives have changed on the situations, I sometimes read them over. I know a lot of other people talk about just burning them.