Not a good sister

Mom, Dad, Heather, and James went to Kim's graduation last night. It was raining and graduation was outside. I asked Kim if i could stay home and she told me yes. If she had told me she wanted me to go I would have went. When I got home my mom was all pissy. I don't know if she was mad that I wasn't going (again they could have told me to go). When mom is mad at something she takes it on pretty much only me and will be nice to everyone else. So, it's hard to tell if I did something or is she just stressed about something else. But, it sucks!