hobo

I went to a friend's (female, lets call her Ax) house for the second time yesterday for her birthday party. This time her sister (attractive, lets call her Bx) was home. She was there, but didn't speak to anyone but Ax and myself. When she did talk me, it was in short spurts, taking advantage of any excuse to talk to me. That plus the way she looked at me suggests to me that she is probably interested in me. I am bad with talking to people that I don't know so although we talked a little, I didn't ask for her e-mail. Odds are I won't see her again until next year (assuming there is another birthday bash and she is there) and I have no other means of communicating to her. Ax doesn't invite people over to her house often so that complicates things.

How can I talk to Bx without making it obvious to Ax that I'm interested in her sister?

Is it appropriate to get involved with a friend's sister? (I don't know how she'd feel about it, but I doubt she would object if we actually started dating)

TIA for anything you have to say.