Wedding Drama - What would you do?

Ok here is the situation…My niece asked my two daughters to be in her wedding and they graciously accepted, but an incident occurred during the Jack/Jill shower that has changed everything. My eleven year old helped with all the preparation ect and at the end of the shower had set aside a balloon because she saw all the other kids taking them home. I was told that I needed to stay and clean up so I did. My daughter set the balloon near my pocketbook and while I was in the kitchen cleaning my sister took the balloon and gave it to her daughter(the bride to be) because it was her day. My thought on this was if she wanted her daughter to have something to take home it was up to her to set something aside. I was very upset by this and I tried to explain this to my sister, but she didn’t want to hear of it. I left pissed off and said this is no way to treat family. My sister then said because of the incident at the shower she would not be paying for my girls to get their hair done. I had never asked her to pay for their hair it was her idea in the first place. Then I get another text message from her saying that my niece is just going to have a maid of honor and a best man. Which is fine, but then I hear that everyone is back in the wedding except my two girls. My niece didn’t have the decency to tell my girls she told her mother who told my mother and I was the one who had to inform my kids. They did nothing wrong and it breaks my heart that they can be so mean. My sister has yet to acknowledge that she hurt my child. My sister is a drama queen and turns everything around to make it about her. The latest text tells me, she doesn’t want us at the wedding. I haven’t heard from the bride she goes through her mother. So should we go or not go?