Crossing some serious lines there, buddy

My fiancé’s sibling is petty. They’re divorced, but everyone else in the family still loves their ex-wife and, as the mother of their children, they can’t just push her out. We invited her to our wedding, and she said yes and was very happy that she got an invite. Well a few days ago, Ken’s sibling texted him saying “I heard you invited my ex-wife. What the fuck is that all about?” A few hours later the ex-wife texted his mom saying she wouldn’t be there. No one even has to ask to know that his sibling told her she couldn’t come. I completely understand not wanting to be around your ex-wife, but when it’s someone else’s wedding, you don’t get to say who gets to come. So now they’re mad, and we’re mad at them, but we can’t make them apologize or make her come anyway. I (foolishly) thought that they would get over themself for one night so that their brother and his wife can be happy. That’s just incredibly immature behavior. It’s not like we’re forcing them to invite her to their wedding.

EDIT

EDIT 2

They absolutely lost their shit on me, I couldn’t explain anything to them, so I turned to their SO to try to give an explanation to why we were upset and a few other things and she totally understood and is going to explain it to them later