Well, I hope Si behaves, both for your sake and for your best friend's sake, as like you said, this is supposed to be a special day all about your best friend and hir renewed vows, not a day about Matt and Si's bad blood between each other. I'm thinking I'd strongly consider removing the kids from the reception environment, although I know that prospect sucks. There may be other occasions where the kids' mutual presence with Matt and Si can be appropriately tested. But now is probably not the time to take any chances.
Even without the kids at the reception, Si could easily confront Matt and push his buttons, so I hope she doesn't do that. She, too, should have the decency toward your best friend to respect the occasion, and not try to hijack it. Guess I'll just keep my fingers crossed for you guys. You deserve a little drama-free happiness.
Love means never having to say, "Put down that meat cleaver!"