How to deal with liking my sister's friend?

Well first whenever your sister invites her over, try to kinda hang out with them, but not like all night! Just kind of 'happen' to walk in the the room that they are in. (This may sound weird) haha. Make sure you get to know her well and she knows you well, because you dont want to walk up to her and say 'i like you' when you and her don't talk. but after yall have talked for awile. Then just tell your sister you like her, and maybe ask your sister to ask her friend if she likes you? And if she does like you, woot woot for you! Just don't get all mushy when your sister is around, that might be weird for her to see. :)