Help? Cancelling plans with a relative?

I asked my cousin if she wanted to go out for a pizza but then my mother reminded me that I had to go to dinner with her and my father . I felt like an awful person for cancelling tha plans with her.I explained the situation to her and she still is upset ( and saying things like now I changed for nothing/I was kinda expecting this ) the thing is this is the first time I ever ditched her so why is she so upset? If I remebered the dinner with my parents I wouldn't have asked her.