A.W.
just try and point out the differences between the big girl and the baby.. my son pulled the same thing when his baby sister was born. I let him go back to diapers for a few months, and then we tried it again. I've spent a lot of time talking about how much I love having a big boy that uses the potty, a big boy that can carry his plate to the sink, a big boy that can help mommy pick up toys, etc. At first he would say "I not a big boy! I baby!" but now he is glad to be a big boy. What got him back on the potty training was putting a sticker chart right next to the potty and letting him pick out his stickers to put on there. He could quickly see how many times he went to the potty that way. And when he filled up enough squares with stickers, I would let him do something extra special. Oddly enough his favorite reward that made him feel like he really was a big boy was letting him pick out his own shoes when we went shopping. hehe.
Now he likes to remind his baby sister (who is only JUST a year old) that HE is a big boy and SHE is a baby who can't use the potty.
good luck though!