This is going to sound maybe a bit weird and unexpected but I think you should let him come. To start, your fiancé's reaction to what Ken would do is perfect; he isn't a family member, Ken would look bad, and your SO could put him in his place. Seems like a really awesome guy, honestly. Nice, kind, and generous but willing to stand up for himself using intelligence. When Ken blows up, it will be bad. His plan is extremely self destructive. I say you let him come, let his plan unfold, and use it as leverage for change in your life. It honestly sounds like he's about to mess up big and that he either needs to drastically change or get out of your life.

I know, I know, it's your wedding. It should totally be drama free. But imagine if it's not. You still get married to the man of your dreams. Your detestable brother looks like a complete moron, yet again. You still get to dance with your dad. You can use the scene your brother makes to take a stand. And maybe, just maybe, you can finally open your parents' eyes and they'll stop supporting him. Or at least justify your actions against him in the future in their eyes as they see him act like an imbecile at your wedding. It'll suck a bit but I really think that if you use the scene in your own way, you'll create a better situation for you and your SO in the future - without Ken.