This is a very difficult situation - one with no clear answer, I'm afraid. What is the nature of your relationship? Are you married - how long have you been together, etc?
For starters, everyone has different ideas about family and what is expected. This can be shaped by your childhood experience, where you grow up, religion, culture, etc. If you and your partner have different basic ideas about how close/intertwined you should be with your families of origin, it might help to clarify what the expectations are. For instance, some people believe that the partner (husband, wife, etc) should always come "first" (i.e. before the needs of others, including family). However, some do not. If your partner falls into the latter category, there may be little movement on this topic - take the time to ask him what he believes and why. Also, examine what your beliefs are, and what shaped them.
Is there any room for you to also be closer with his family members? Do you have a good relationship with them? Would you like to? Ask your partner how you can also get involved in events with his family so that he will begin to see you as one unit. Remember: It isn't you vs. them. If you set up that dynamic (e.g. "It's them or me!"), the relationship won't work. That will only cause him to defend his family members more staunchly and you risk being seen as an enemy. Your best bet is to try to figure out a way to value them as well.
To clarify: this doesn't mean you always have to believe that what his family members do is "right," but it may help to try to display more empathy toward them - to help your partner in helping them. It puts you all on the same side of the issue. That way he doesn't have to choose between you and his family.
Please let me know if you have further questions.
Dr. Greene here...just checking in to see if you're alright. You sounded frustrated in your initial question and I wanted to see how you were.