When my ex-husband wouldn't set boundries with his manipulating ex-wife, she turned their children against ME because I was the one who said ENOUGH! It turned me into the bad guy and from that point on, the kids when left in my care, would not mind me, would say, "I don't have to listen to you, you're not my mom", etc. All because my weak, ex-husband (their father) wouldn't set his own boundries with his ex-wife and I was left to do it.

I agree with Byz. Your BF needs to establish some ground rules with his EX now before this relationship is finalized. His children need to understand that they will always have a place in your lives like _sabby_ said, but they also need to understand that their father is choosing to begin a new life and an additional family with YOU. I can almost guarantee that the EX is behind all of this. It's up to your BF to sort this out before you make any more wedding plans or I'm betting you will regret it. Being the negotiator is not fun and doesn't work.