Mother-in-law let my daughter down

My daughter is a senior in high school and is in the marching band. She had an event yesterday that is the end-of-the-year performance for friends and family. It is a big deal, people take pictures with their families, etc. Anyway, I was working that night and my husband had to take our other daughter to another event, so I asked my in-laws if they could go and support our daughter. They said they would. I was happy about it because I wanted someone to be there to celebrate her last performance, support her, etc. I am so so proud of her.

But today my mother-in-law texted me and said she wasn’t able to go. She said they had family visiting and also had a headache and couldn’t go. This is a pattern that they tend to have. I think I knew in my heart they weren’t going to go. I’m angry because I feel they let my daughter down. They didn’t go to a single one of her performances all year and then on the one where I wanted to have someone support her they didn’t go. I know that I have a manual for them that they are not following, but I can’t figure a way out of these angry feelings.

Here is my model:

C: In-laws did not attend end-of-year band performance
T: They let her down again and they shouldn’t have done that.
F: Angry
A: Judge them, find evidence for all the ways they aren’t showing up for my kids, draw attention to their flaws
R: I grow more anger in my heart. I let myself down (?)

I want to honor my feelings of anger, but I also realize I don’t want to stay here forever.

The best I can do right now is a thought like: They are flawed people and that is ok. Please help.