Hugs sweetie.
I know you want to rush in and make things better, and that you hate seeing SYB in pain- but it's his family and he has to make it work. If they're (the two sisters) choosing to keep the rift going, then they're going to... and since they seem to enjoy the drama of it in their lives, it's not likely that they're going to change.
As you know, my brother reached out after 15 yrs to reconnect. It took me a few days to work through the pain of our history together and decide that I could chose to be happy rather than right. Doesn't erase the history, and I will take things slow, but I'd rather have him in my life than stay upset.
My point is that if SYB is waiting for an apology for their past behaviour, he may never get one. Better to just release them from any expectations and move forward: expect nothing and be pleasantly surprised when you receive something.
As for what do YOU say? "I stand behind SYB's decision, whatever he feels is best."
Go to the reunion, smile, keep your mouth shut and be support for SYB.