I don't think there's anything wrong with your idea to not go to the event…although I know it must seem daunting to try to explain this to your family. Perhaps you could explain only to your sister, since she sounds supportive, and just state to the rest of the family that you will be unable to attend? You aren't really obligated to provide a detailed explanation to the rest of your family. I know it's more easily said than done but try not to engage in a debate about why you're not going…just calmly say you can't make it and leave it at that.

Is there a way for you and your sister to get together privately to celebrate? I'm sure you already know this, but graduation ceremonies in general are a recital of long speeches followed by a slow reading of hundreds/thousands of strangers' names. The family sits in one place and the graduate in another…so all this will take place without your sister present. Then the graduate finally sees their family afterwards--that's the part where the family says "congrats" and supports the graduate and probably grabs dinner.

You won't really be supporting your sister by sitting through a long ceremony without her present while panicking next to your abuser. I don't think you need to feel guilty. Tell your sister the truth: you are very excited about her special day and you want to congratulate her in person. Then figure out a time to celebrate together.