Yes, actually, I agree with Shellbell and Tobily. It's important to let her talk about how she feels. I didn't actually feel disappointment straight away but once I left the hospital, and particularly after hearing some of my ante-natal buddies more positive birth experiences, I really started greiving over my lost birth experience. One of my ante-natal friends described her caesar was a positive experience, having the staff in theatre chatting to her, cracking jokes and making her feel at ease - then being able to hold her bubba in theatre. I started to feel a little sad and ripped off cause my caesar was under a general and I missed out on that.
Hopefully your sisters caesar was a positive experience like my friends was, but it is only natural to feel ripped off about not experiencing a vaginal birth.
She is not inferior or a failure for not birthing vaginally though - make sure she knows that.