okaaye, theres 2 possible thing this could mean.
either A) she likes you
or B) she likes annoying the hell outta you.
i would say she probly does like you though, im not sure she wants to show it too much depending upon how her brother would react. if you like her you should find someone to tell her or give her hints. and if u to decide u want to date..make sure you tlk to yur friend about it before seeing his sister. if you dont like her theres not much you can do but tell her toback off..hope this helps[: