gather all your siblings and have your picture taken with santa,,,,,I don't care how old you are or how young...your mom will love it
maybe you can make her a gift or write her something like a song or poem because if it comes from your heart she'll probably love it even more (plus it wont cost as much as earrings) but if that doesn't work how about just buying her matching necklaces or bracelets to go with the earrings you bought her.
==========================================
Okay, so I don't have the perfect gift idea. It is very hard to shop for your mom. Do something that is personal and isn't about money and then also give her some gift that she always uses, like a nice purse or coat or something you know that she always likes to have new or really classy.