I wasn't adopted in the legal sense of the word. I was lucky because I grew up with my sister. Our grandparents "adopted" us after my mother died when I was 5 years old. My sister (actually a half-sister: we have different dads) doesn't know her siblings from her dad's side or her father. It isn't due to her not wishing to know him. There are complicated circumstances there that I don't feel right explaining.
As for myself, I have a half brother (and apparently, a few other siblings which I don't know anything about) from my father's side of the family. I have never been where you are, but I can relate to sibling situations. My brother seems to not want anything to do with me (or get to know me) and is a general pain in the butt. I can say from experience that you need to talk to her...not cruelly, but definitely in an honest manner. It hurts me that my brother acts like he does because I have tried countless times to get to know him and his kids. If, you are unable to be a big sister to her in EVERY sense of the word, you need to tell her that. Maybe someday you will want to get to know her more. You don't want to burn your bridges now...either by acting apathetic or by being distant on purpose. I can tell you that my brother has pretty much burned his bridge with me. It is sad but true. A small part of me still wants to hang on...but only a small part of me (and that is more for my nephews rather than my brother).
I'm thankful for my sister though. We used to be practically strangers despite being raised in the same house for years. That has changed for the better though. My point is...you never know what the future might bring.
Sorry for the novel. I didn't mean to make this reply so long. I hope it helps some.