Annoyed

So my sons birthday party is July 3rd. My mother in law texted me saying she wanted to help and I asked her to make two dishes and she was like yeah I can do that but I wanna do more then that and at the time I didn’t really have any plans on what I was doing yet so I told her I would let her know. Then she told my husband after we talked to tell me she can do more than just make the food I told her to which kind of annoyed me because I told her I would let her know because I didn’t have any plans at the time. Fast forward I have already bought all the decorations and spent about $400 and the cake was $100 which my grandma paid for. So since she said she wanted to do more we all made plans to go to bjs for all the food which she is the only one to have a membership for. We made plans to go two weeks ago on the Saturday before the party and she was gonna he’ll pay for the food. It’s Wednesday now and today she told my husband she made plans to go to the beach and can’t go to bjs anymore so now I’m super annoyed. She knew we had plans to do this and she makes plans to go to the beach??? So now we can’t go because it’s her membership and now she won’t be helping pay for the food since she won’t be there.. I don’t mind paying for my sons party I didn’t expect anyone except for my mom to help but she offered! So that’s why I’m annoyed now. My mom told my grandma because she has a Sams club membership so now my grandma has to go with us and is helping us pay (I appreciate her so much!) but the whole situation annoyed me. So if she complains about me not having her do more I’m gonna be pissed! My husband agrees with me too but neither of us will say anything to her lol