Rylee, my 2 year old daughter, is starting to act more like a baby. I thought it was because she…

Rylee, my 2 year old daughter, is starting to act more like a baby. I thought it was because she knows she won't be the baby anymore but now I see its because she is around my sister's daughter who just turned 1. Does anyone have any advice on how to get her to break out of trying to be a baby? The whinning and baby talk is killing me because before she talked so clear and only really whinned when she was sleepy.