I had the same problem with my now 2.5 year old. When she was around your daughter's age we went to the local LBS and let her pick out her own helmet. It made a difference because it was her own and not a handme down from her sister. She felt some pride and ownership in it...

All the other suggestions are on point. There is no option to not wearing it...wear it or stay home. You and your son head out on a ride while she stays with Dad or a neighbor. It will make a point. Its amazing how fast they will come around as long as you are consistant.

I am sure you have tried the snacks, books, etc....
You might try making a list of things she has to find on a ride... bird, red car, leaf, Truck, Flag, garbage can, etc.... I used to make lists and draw the pictures next to the word so she knew what she was looking for. Works for scavenger hunts too. Hope it all works out! She will be riding two wheels in no time!
My youngest (2.5) just got on the 12in. w/ training wheels...no stopping her now! Have fun!