Extra special gift for bridesmaids

Both my sisters are my bridesmaids. We have always been extremely close so it was only right they would be coming down the aisle with me. I really want to get them both a more sentimental gift that also represents us joined as sisters as well as thanking them for being my bridesmaids. I've looked at lots of jewelery and thought about maybe rings with our three birthstones on (very small and dainty) and then they can wear it on the day (might get myself one to match too) but then I have the issue of trying to guess what finger they'd want it on and their sizes etc.
So just posting on here to see if anyone has any ideas?? Thanks