My father in law avoids us , how can we fix it?

You can't fix it and you know it. Perhaps someday he will realize that he was wrong and he will ask you to forgive him. But until then, leave it alone. Don't stress over him avoiding you because it's not worth it. The only thing that is important is your husband and your family. The day will come when he will realize that what he did was wrong.

you could try apologizing for whatever you think you mightve done wrong if you think it can help

You've asked a bunch of questions about your father in law and sister in law, but not once have you mentioned your husband. He's the key to all of this, because it's his dad. It's hard to answer without knowing his role.