It's really difficult when your partner's family dislikes you. That kind of strain can take a toll on the relationship. Plus, it sounds like the fact that his father recently came back into his life is making his father's opinion of you a little more valuable to him on some level, because he probably doesn't want to lose touch with his dad again. The one time he did briefly break up with you is probably making you really anxious that it could happen again.
Here's some advice you can try: First of all, allow them to dislike you -- but turn the other cheek and you be nice and gracious to them in spite of their dislike. You're going to have to win them over.
Not everyone likes people right of the bat, and many in-laws don't get along with their kids' spouses, so don't dig in and fight back -- instead, kill them with kindness. If your boyfriend sees you're making an effort in spite of their bad behavior and dislike of you, he's going to be more inclined to take an active role in protecting the relationship he has with you. It's important for your relationship with him to be strong and face his father and sister with a smile and a good nature. Let their dislike of you be their problem -- not yours. When you give their dislike of you weight, it becomes important. If you can laugh it off and focus on what's good, it will fade away.
Next, lower your expectations. Your'e probably very nice and you have lots of friends and aren't used to people disliking you. Consider yourself lucky -- and stepping into the "real world" where sometimes people don't like you -- because of who they are and what's going on in their lives -- not because of anything you've done. Try to feel empathy, and don't expect a glowing reception. If you can do that, you won't be so disappointed.
Hope that helps!
April Masini is a relationship and etiquette expert, author of 4 books and the #1 relationship advice forum, blue-chip corporate spokesperson and preferred go-to source for the world’s most prominent media and news outlets.