What would you think or do in this family sibling dispute?

I am the oldest of 3 siblings in the family. Our mother recently moved back to our hometown "to be close to the kids" again. The youngest sibling, my sister, is 23. She decided to have a very elaborate and expensive dinner for our mother, that doubled as a birthday party for her. I was invited, and was told not to bring anything. I decided to at least show up with a birthday card, and also purchased a small shopping card as a gift. The meal was at my brothers house, and it went well. But after getting home, I received a text message from my sister, demanding I pay 1/3 of the cost ($150) of the meal. This was upsetting, as there had been no communication beforehand that I was expected to help with the cost. My sister also refuses to acknowledge the gift card I purchased towards the cost of the meal, or what she feels I owe. I am not a cheap person, but feel it should have been communicated beforehand that my siblings expected me to pitch in. And shouldn't they acknowledge the gift card in some way? Wonder what others think or would do in this situation?