fsil boyfriend drama

I cannot believe that here I am again posting for third party advice on another issue brought up by my FH’s family… I have already cried and I’m just not even feeling excited about my own wedding anymore. His sister broke up with her live in boyfriend of barely a year about a month ago. Right about the time that we found out they might be breaking up is when I was having the invitations made. I asked my FH to confirm with his family and I even confirmed with them about wether to put her soon to be ex’s name on the invitation. The answer was to no and to make sure it was sent to his parents house and not their apartment together. Okay so done deal. She breaks up with him and there was a lot of drama, moved out of their apartment and everything. Fast forward ONE month later my FH finds out from their dad that my FSIL and her ex are back together. Mind you my FH cannot stand her boyfriend and his parents don’t like him either after he showed his true colors during their breakup. Now within this same conversation with his dad about finding out their back together he gets asked if her ex can still go to the wedding and that his parents will pay for him. We are ONE MONTH out from our wedding btw. We simply say no. There’s too many details that have already been finalized as well as both of us just not even wanting him there. We think it’s ridiculous to expect us to just accommodate the change so quickly after all of the drama that took place during the breakup. His parents admitted that they do not like him and are only pushing it on us because that’s what his sister wants. I cannot believe this has become my reality…