LOL! And I thought only my family acted like this! My husband's family all lives near us and there are 4 aunts, around 20 "kids" ranging in age from 2-30, and grandparents. And everyone likes to go out to dinner and buy gifts for EVERY birthday. With dinner and even a modest gift, that adds up to $1000's every year. We've tried to get everyone to stop doing this because there are only 3 kids left under 15, many in the family are on weight loss programs, so shouldn't be eating out, or having cake....it's all just silly. But everyone keeps having their bad habit and gets bent out of shape if you don't show up for these things. Some months we're out to dinner 4-5 nights in a row because of lots of birthdays, it's NUTS!
We're working really hard this year to do a once a month family birthday celebration. No out of town, no dinner out. Just a rotating potluck once a month at someone's house, no gift giving except to young kids and celebrating all the birthdays and anniversaries for that month all at once and enjoying one anothers company at home, not in a noisy restaurant. Many are still breaking the rules, but we're doing better.
It's not crazy to abstain, try and work with your sisters to find a better way to celebrate one another and get together without the cost and the hassle. Old habits die hard, but it's worth it!