i was in the same spot

a mate of mine (not a realllly good mate, but a mate non the less) had a sister who showed intrest in me, and to be honest he didnt seem bothered at all, it did change the way we had a laugh together etc, it still has affected it, but he was cool with it and would even give me and her lifts to see eachother and stuff! so not all to bad really.

good luck, i'd suggest saying to your mate something like, 'would you mind if i go and talk to your sister' see if he protective over her or not?