I think your being unfair to get between your b/f and his ex, because weird as it is to you, its none of your business. It works for them and you should leave him and his family alone to be happy on their own.
I think the main reason he even is considering your side is to keep peace with you, as he does his wife, and family. You really are putting him in some very unfair positions.
Build your own happiness around your family you have there between you and leave his alone before it gets messy and all the kids will suffer.