HELP! I'm falling for someone I shouldn't!

I'm in a major pickle right now. I'm falling for my best friends sisters ex. The sister and I are good friends too, so when I met the guy she was talking to I was supportive. Well, after they broke it off a little over a month ago he started texting me. I texted back not thinking anything of it, but now things have changed. We've hung out 2 times since they split, and I'm going to see him on Halloween. I feel awful! I feel like I betrayed my friend in the worst possible way. What should I do?