Long and complicated question dealing with my father and siblings. How do you think this should be solved?

Your siblings have told you how they feel--respect them and their feelings. You are only responsible for your own behavior and your own decisions. If you want to keep seeing your dad and you want to have a relationship with him and his family then I think it is a good idea. However, your siblings don't feel the same way. They don't have to feel the same way. For your sanity and peace of mind decide to let it go and stop trying to make everybody one big happy family. I know you want to try and make your unhappy childhood have a happy ending, but you can't.