I must confess it is disrespectful to be on a date or an outing with a girl and be starring at another girl, Best bet is for you to break up and go after the other girl.
Sometimes when i go out , i see a guy with another girl starring at me or my friends, Even if she is your sister , she also deserves some level of respect from you.
More than the guy, i respect the girl that is still moving with that kind of man because to me i see it as outright Disrespect
