The best thing to do is handle it on a case by case basis. Try to see things from the siblings' point of view.
Establish a guideline, and if by that point you have invested time and energy into the situation and the girl's brothers have not taken a liking to you, or her family has not, then determine if the effort you are putting forth deserves continued commitment. If not, then cut your losses, and move on to another girl.
Another thing you can do, is determine if there is some merit behind what the siblings are thinking or the parents are thinking. Are you really worth going out with their daughter or sister. Who are you, and what do you have that makes you worth anything to them or your sister. Maybe they feel that their sister or daughter can do better than you. If so, maybe you should accept that, and gracefully exit. You'll be doing her a whole lot of justice, and probably win everyone's respect.
