They can't force you to go. You do have a choice here.
I had a disagreement with my grandmother last year and have not gone to family gatherings since then. While I do plan to reconcile with her in the near future, I put my foot down on this and told everyone that it would just be too uncomfortable for me and I was skipping out on it for the sake of my own well-being. Perhaps you will need to do something similar.
That said, I would personally try to continue going to family events given what you've said... family is important and often an excellent support. However, you do need to establish some boundaries and stand firm on them. What I would do next time is ask that certain topics or questions be off-limits and state that you will not come unless people agree to this. If they do not agree, don't go, no matter how much they try to persuade you or guilt trip you. If they bring up those topics after agreeing, firmly remind them. If that doesn't work, excuse yourself from the party temporarily or just leave entirely.
That should help make it clear how you feel and how seriously you take it. I would also ask your grandmother and cousin for advice!
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Yes, I have autism. No, it isn't "part of me". Yes, I hate my autism. No, I don't hate myself.