Haha, no idea why you would risk a friendship over a girl especially when you're young so you meet new people frequently.
I would say no because:
1) Risk losing the friendship with your friend.
2) If it's messy it's even worse.
3) Your friend could get jealous
4) I agree it shows a lack of respect- it almost shows as if nothing is out of your bounds
5) If you're young you probably meet loads of people so if it's casual and his sister gets attached it won't turn out well.