So, I had a party with one of my friends because it was his birthday. He has a sister and we talked before and partied before but we didn't interacted much. After the party we sat in the same car next to each other and had a little small talk and flirting as well ("she insisted I wasn't in love with her and I just called her out on doing it on purpose because she fell in love with me, she blushed a little). Now we did vibe pretty well and I want to get to know her better. Problem are those: I don't have her number (yet) and she is the sister of a close friend of mine.

Should I just pay my friend a visit and try to get closer to her that way or do I just ask her out on Instagram?