Am I wrong? Moms birthday...?

Alright so my mom didn't have a great marriage and isn't real close to her family and feels left out alot etc. She says she has never had a good "name any holiday" and her birthday is coming up. Well her best friend started a cake business and never received training. Since its my moms best friend we bought our wedding cake from her, and lets just say she butchered it. We still payed full price was nice and all etc. I even asked her after my cake if she thought by January she would be able to make a coke cake for my mom... obviously I'm throwing her a party if I'm buying a cake?

She has been bugging me since before Christmas for details about my moms party, which I didn't have planned given the guests intended wouldn't have their work schedules for me to make reservations. My mom changed her mind about what she wanted for the cake and wanted a coca cola/beach theme, hard to pull off without looking tacky but I spent a most of the day figuring out how and came up with some really great concepts. Her best friend doesn't want to/can't make the cake I sent her and is giving me cakes I don't like at all... she also has been inviting guests without talking to me, and I heard about it because she was talking to an aunt about how she was planning the party.... She is planning and I'm paying!?

I'm pissed and told her that we could do seperate parties. Her vision is so different from mine and as my moms best friend you would think she would donate the cake, but I'm paying for it and she wants to do whatever design she wants.... When I suggested seperate parties my sister called me selfish and she backed out altogether saying she doesn't have the time or money to plan a party!

I mean am I wrong to expect that if I'm supposedly planning and paying for the party that I am the only one inviting guests, choosing the cake, decor, etc. My sister said I should just let her do the cake, but she won't do what I want and I'm paying for it, then my sister offered to pay for it but if the cake doesn't match the decorations then its tacky, plus the cake ideas she showed me I hated and I don't feel like they would be my moms 1st choice...