Family Fight? Should I feel bad?

So! Yesterday was my mom's birthday. And for her birthday, we where going to out to dinner, My mom, dad, 3 brothers and I. But yesterday morning, my grandmother called and said "i'm crashing your party. I hope you don't mind" And my mom really didn't care that she was coming, because she was going to bring her mother (my mom's grandma who she hasn't seen in a while) so she didn't have a problem with it. Now, don't get me wrong, I love my grandma to death, she has done so much for us. But, she is the type, who wants to get all the attention by throwing her money around. And if you suggest how she spends her money, she will do the complete opposite. But anyways, we had no problem with her coming, until she called again about 30 minutes later. She told my mom, that she was going to bring my cousin and his new soon to be wife. Me and his girlfriend do not get along at all. When I was about 15 ( I am almost 17 now) she told me I was fat and needed to go puke in a toilet, right in front of my cousin! Of course when I told my grandma about this (because it was at her house) She freaks out on me, saying it was just a joke, and blah blah. So I decided to let it go, until the last time we all went out to eat, for my little brothers birthday and again she called me fat. So, me being me I freaked out, and started yelling and calling her a bunch of bad names, and since then we haven't talked. So I texted my grandma, and asked her why she was bringing my cousin and his girlfriend (also I don't have a problem with my cousin, I love him to death) And she called me, and freaked out on me, saying I have an attitude? And all this! I was so mad, that she would pick my cousins girlfriend, rather than her own granddaughter (also my grandma has favorites, and picks my aunt over my mom any day, and same with my aunts kids) But anyways while on the phone, I told her that if his girlfriend comes, I refuse to go, and if I do go, and she gives me one dirty look I was going to get up and punch her in the face. And once again, she got all offensive and stuck up for his girlfriend, but in the end, she said she wasn't even going to come anymore, and it would be just my side of the family. Was I wrong to get mad that she was going to bring her? Or do I have every right to be. I feel so bad, because my mom ended up in a little fight with my grandma also ): But in the first place, it was just soppost to be my side of the family. Sorry this is soo long!