Cousin reneged on verbal contract?

originally wanted to plan a party for one of my sisters for a sweet 16(I am her legal guardian;her twin lives with my cousin). Listening to family, I decided to include my cousin on the planning . I did most of the legwork(deposits,buying decorations,keeping contact with dj photographer venue)and my cousin just got the table decorations done.She is upset that I invited my GRANDMOTHER to the birthday party, called me, and decided to pull out the party. Not only is she not coming up with the money she intended to pay , but party isnt for the other sister anymore. Can she be sued for her half of the expenses that I now have to pay to keep the party plans rolling?($700) The party is next weekend.She pulled out on the party today . I am aware I wont see the money for a while. We are also not on speaking terms.Oh, her reason for not liking my grandmother is due to family RUMORS that my grandmother didnt know about. Now I have to face telling her why we wont be seeing her other granddaughter