OK, I guess I'm going to be the lone voice again.
I think a 50th wedding anniversary trumps a child's 14th birthday party.
Technically, they were rude to accept the invitation. However, real life often demands that people are more flexible than the rules of etiquette dictate we should be. The anniversary was a once-in-a-lifetime occasion. A child's birthday comes around every year, and unless there's some cultural thing that I'm not aware of, a 14th birthday isn't anything 'special', like turning 18 or 21 would be. By attending both events, I think MIL did the best she could in a difficult situation that she was placed into, and she didn't blow off the party to attend the anniversary party.
Your feelings are your feelings though, and if you don't want to throw any more parties off the back of that it's understandable, but I'd cut MIL some slack unless there's some kind of toxic history there.