You have to come to realistic thinking to avoid negative feelings with whom you remain obsessed. Try to understand others from their perspective. Don't try to be judgmental during any discussion with others. Try to avoid argument with others and try to convey your perspective in an effective manner. There are a nummber of girls who are single child or don't have sister so you are not alone in this world. Try to make friends with whom you can share your feelings and thoughts for catharsis and to spend a valuable time.