I’m so very sorry about this situation. I think you and your husband already did the best thing you could do. You moved out of a situation that was hurting you. Now at least you don’t have to deal with your sister-in-law every day. It’s unfair and sad that your father-in-law now believes negative things about you. I do worry that he is being taken advantage of. But your husband is the one to try to talk to him about this, not you.
Your husband wants to visit only on special days. He isn’t ready to give up on his family. I understand that. But I hope he is able to provide you with some protection while you are there. I also hope you can learn to separate your feelings from what these people say. You know their words are not true. You know they are not your friends. You do not have to let their hurtful words hurt you. They are only words. They don’t mean anything. Talk to your husband ahead of time about what you need him to do to help you manage the visit. Plan things you can do or a place you can go to get a break if the situation gets difficult for you. Maybe you can bring a book or some sewing to work on or maybe you can go for a walk to break up the time. Keep the visits as pleasant — and short — as you can. Your husband can then meet family obligations. And you limit how much time you have to manage a toxic situation.
I wish you well.