First of all, wait until she’s of legal age to date her. 😉
Second of all, talk to your sister before you ask her out. I’m sure your sister knows you like her, but it’s probably a good idea to give her a head’s up. You don’t have to ask permission, but it’s a nice show of respect to let her know you’re going to ask her friend out and you hope it won’t get too weird between you all.
I hope that helps! Let me know how it goes, and please follow me @AskAprilcom on Twitter and on Facebook at this link: [url][/url]. 😀