"Sisters, sisters There were never such devoted sisters, Never had to have a chaperone, no sir, I'm there to keep my eye on her Caring, sharing Every little thing that we are wearing When a certain gentleman arrived from rome She wore the dress, and I stayed home All kinds of weather, we stick together The same in the rain and sun Two different faces, but in tight places We think and we act as one Those who've seen us Know that not a thing could come between us Many men have tried to split us up, but no one can Lord help the mister who comes between me and my sister And lord help the sister, who comes between me and my man"
Try not to let the snarks in the thread distract you. Best of luck to you and your sister.
Also, kind of cheesy, but "You're the Top" by Cole Porter, cause if she's having trouble fitting in she might need a self-confidence boost, and it implies that you think she's the best.
This song, not mentioning sisters but rather friends, isn't strictly speaking, apt, but if her situation isn't too dire and you're looking to let your sister know you support her, love her, and are there for her, this could fit the bill. Might even make her laugh.