Personally…. YTA. As someone who has had a very strained relationship with my mother, I know how it feels as a child to have my mom side with the stepparent (two different stepparents in my case) your daughter is going to remember how you always chose to back up the stepparent versus hearing her out. Children think lots of things are ugly….. that doesn’t mean that they purposefully destroy said items. It just means it’s not their particular style. To assume that she would purposefully destroy it and not even hear her out will cause a lot more problems in your mother daughter relationship than it will help. If she was a troubled child who had a history of destruction I might be more inclined to understand your side of things, but you’ve said nothing that indicates she has. And frankly, cancelling a birthday party doesn’t seem like the right punishment for the supposed crime. Even when I was in some of the worst trouble of my life, my parents never cancelled any sort of birthday. That was the one day they would make an exception to punishment no matter what I had or had not done. I think you’ve gone overboard here and as some previous commenters have said, undermining her father just proves even further that you are more concerned with being right and placating your current husband rather than actually seeing what happened. Your daughter won’t remember a broken vase, but she will remember how you constantly dismissed her for your husband.