To have a relationship with my dad or lose my sister?

We learn how to be parents from our parents. Your father did what he thought was right even if it wasn't. Your ability to forgive him shows that you want to move on and let the past be the past. Your sister wants to hold on to the hate she feels, I believe that this is her way of protecting you and trying to pay you back for not being able to stop the abuse when it was happening. I think she feels some level of guilt and is taking some responsibility for what happened to you. Tell her that it was not her fault and that you don't hold her responsible for what happened to you. Let her know that you love her and that this will not change anything that you and her share together. This is just an opportunity to have the relationship you didn't have with your parents when you were young. I think that she may understand and you can help her to find forgiveness. You can both do this together. Best Wishes.