Well, IMO, there's a time and a place for this stuff. (Somewhere on here there's a thread from last fall, when my dad died, I had to speak at the funeral AND my abuser who few know the identity of was going to be there. AND, I've "confronted" him in a letter that he ignored.....) I survived (obviously). Got a lot of good support and insight from people here. It went pretty well, all things considered.
This is your sister's graduation. IMO the day is about THAT. It's not about you. It's not about your abuser. It's about her. That's just MY OPINION. It's a point of view.
There may or may not be a point in confronting him,. That might best be a battle for another day. A day that can be about THAT.
My T chooses to focus on how are things going NOW and how can we get them to work better down the road. That works for me. It fits with how I see the world. He doesn't seem to think i need to confront anyone. If I wanted to, I'm sure he'd help me with it. What he says about abusers is, they pretty much don't get it. They don't see it as "abuse" and they don't get what all the fuss is about. They really, truly, purely don't. Kind of hard to believe when you're on this side of the issue because it darn sure looks like a big deal from HERE. To them, though, it's usually not and most likely never will be. Which means, "confronting" is fine, if YOU will get something out of it, but don't expect it to change them or affect them in any meaningful way. Don't expect an apology. Don't expect insight.
You really don't have to PRETEND anything. Things are what they are. No, what that person did was NOT ok. But YOU are going on with your life, regardless. And you're capable of being the kind of person who puts their own hurts aside on a day, to honor the sister you love. YOU can chose the time and place for a confrontation, if you want one. You're not denying anything by deliberately choosing not make your sister's graduation into a drama about you and the abuser.
Again JMO! Think it through and come up with the path that works for you. Different paths for different people. And good luck with what ever path you chose. I'm pretty sure it won't be an easy day for you, no matter what.