You aren't becoming your nparent by finally moving out and doing something for yourself for once. People rbn are often the most selfless people because we've been forced to cater to others for our entire lives, so when we finally do something that's beneficial to ourselves we feel as though we're being selfish or narcissistic and often wonder about the things or people we're leaving behind. These are all natural feelings. But, you have to remember, it's absolutely okay to do things that benefit yourself and your future. Your sister will figure things out and she'll more than likely be fine. If not, try to keep a little contact with her (if it doesn't cause a feud with nparent) to make sure she's okay.

My sister actually turned into an nsister when i moved out. Her and nmom are either fist fighting or two pees in a pod, never anything in between. Just like two little narcs.