First of all, it's really bad for a girl to socialize too much. As a brother, she should give her sister the right guidance. First of all, have a good talk with my sister and find out what some girls are thinking. There must be a reason why my sister promiscuous with others. Is it insecurity or lack of love since childhood? Still spoiled and willful? You need to know these things first. Then tell your sister what you think and how important it is for a girl to lead an honest and clean life. In addition, see her more recently, spend more time with her, and do some meaningful things with her, such as studying, reading, exercising, shopping ... as far as possible, let her have no time to contact her former friends! Encourage and praise her to do better things, so that she has a sense of accomplishment. One more thing, you'd better not keep these things from your parents, but discuss them. You are the people who love your sister the most, and your help is the most important to your sister!