Sounds like you are more interested in using your dress than she is so I would be very careful about cutting into that. If you really want to use your dress though, perhaps do something for yourself to use at the wedding, such as an evening wrap? Be careful that you don't upstage the bride though! Or perhaps use in table runners for the reception that you can take to your own home to enjoy. In years to come, she might find those have value more than right now.