Money is always a tough subject. I would do one of two things: 1. Tell her that your airfare is a large expense and that you can only afford to contribute $xx.
2. Ask that your airfare be included in the cost of the shower and then divided between everyone paying for the shower. (My family did this for a family reunion in NJ - half the family was here and the rest had airfare and we all split everything - everyone was happy to share the expense!)