Problematic situation

So this weekend my sister, 22, came home from college w 2 friends... One of her friends had an obvious crush on me from the get... She pretty cute too tbh.. I didn't do nothing (going against all the testosterone in my body).. Bc well I just see the whole situ being bad in the long run considering my sis lives w this girl.. But now, despite being back at school she got my number n is continuing to text me.. N I don't wanna b rude, n I also want to hit it... So I'm like ugh... I don't know what to do w this one...