What should I do about this situation with my fiance?

You should talk to him with regard to you feeling like an outsider. If you are going to be married you should be able to communicate these things. I would o to the graduation for sure. Even though you dont want to, relationships are give and take. You mother is kind of right but i would leave it up to your fiance to put his sister on her place. He needs to be clear that you are who he chooses and she must accept that. Maybe you could have a cake ready for him when the two of you get home afterward, that could be very romantic especially if you run in the house before him to light the candles :o) good luck!