How can I communicate with a friend’s sister/her friends?

When I hang out with friends, sometimes one of them bring their younger sister and her friends. For context we are all around 21-25. I'm always a bit unsure of how to talk casually with them. I always greet them say bye etc, but chatting casually just seems to come off as awkward when compared to other people not related but around the same age or younger. Is it just because of the relation with my friend as his sister or something else? Always felt a bit odd especially when she seems to get along well with all our other friends. Maybe it's just I don't know her for as long as others? I wanna chat with her friends casually too but they are always together