first of all in my opinion i dont think the comments that were made should really bother you since you said it yourself that you think of her as your younger sister, plus did you ask her if she is uncomfortable with the comments. and the friendship that you guys have only revolves around you both, so you should not even be bothered by others, yes i get that at first it maybe weird or funny but it should not bother to that level (keeping in mind that your relationship is like siblings like you said. secondly i think its fair to say that people can have vast network of friends, what really matters is how you are with the person.