Why am I so mad even though I know I have the right to be?

My sister and her husband have started to hang out more with his family. I have always tried my best to be in my nieces life and have also taken care of them since they were young. Well it was my niece's birthday and I got dressed up and had told my mom one month ago and so we could go. I didn't leave with my sister because I was supposed to go with my mom. I don't have a car so we were going to go together in her car. We're the only ones on my sister's side. My mom doesn't get along with my sister's husband and his family but I have tried to get her to forget them and let's just be there. Well today I'm already dressed and my mom comes in and tells me at what time is it. I had called her twice and she didn't pick up but I didn't know she went to go do an errand. I said it's now but she wasn't dressed and when I told her she goes well it's past the time. I text my sister we're not going but if she can come pick me up and I can be there. My sister tells me I'm sorry but I can't because I'm already sitting here and some people are here.

Updates
2 mo
It seems my whole life I have had to try to get a family even though I already have one. My dad abandoned my family since I was young so I have always felt I have a broken family. So when I am trying to keep the little family that I do have. I have always seen just one person as family but I can't even get that. My mom than went and tried to make it up by telling them we'd go there again, and I told her not to worry about it.
Updates
2 mo
I don't get it you say we need to stick together and as the grandma you don't go even though you got the day off. I'm guessing it's also the representation of my family I'm trying but I'm getting to the part of not caring either. Maybe this is the reason why people get with other friends. I'm just trying to keep things together and I'm obviously not good at it.
Why am I so mad even though I know I have the right to be?
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