A.F.
I don't think you should worry.
I have a friend with triplet daughters and they tend to only want to socialize with their sisters. While they are only 3, they do tend to not let other little girls into their circle. Not intentionally, it's just what they are comfortable with and used to. Maybe she's content to be close to her sisters? I've also experienced this with sisters who were friends as a teen.
I would just keep encouraging her to form bonds with other people so that she's not socially isolated later for any reason. She also may just not be to the point where she's interested in that yet. She sounds really well rounded.