Can Anyone Else Figure Her Out?

So this girl is throwing me mixed signals...I think. She's my best friends little sister (I'm a junior in high school, she's in 8th grade) When I hangout with my friend she ususlly keeps her distance, but sometimes she'll hang out with us. When she does she sits relatively close to me and often talks to me (directly to me, not her brother) about humorous happenings at school or between her friends. Other times she completely ignores me or at least avoids me. Does she like me? How do I respond to that?