Unfortunately I have another sibling who flatly refuses to go to a children-only gift exchange, because she wants her present from Mom. I may wear her down yet, though.
I feel your pain--as we've been there, done that. Out of 4 siblings, DH and I finally put our foot down this year and declared no more gifts excluding children under 18, any more. His other three siblings didn't want to do that, so they didn't. However, my SIL announced that this would be their last year and they are going to it next year as well.
Simply put, if you don't want to, don't let your siblings wants dictate your actions any more.
I promise you, next year you will be SO GLAD!