Our expert says:
You are in a very difficult position, and ultimately all you can do is give her mother and her the benefit of your advice. Warning your sister of the dangers involved and also trying to help her see that offering sex or sexualised behaviour will not provide her with a boyfriend or provide true friends is an important message and one you may have to repeat many times for the message to get through. Where her mother is concerned you may want to let her know of your teen experiences and point out that your sister is going beyond this and this will lead to major problems in adolescence and adulthood. I would also be clear with your dad about what is going on and give him the benefit of a young woman's advice on where this could lead for her.
I would recommend she has therapy but she may be very reluctant to take up such an offer. Keep raising your concerns each time something happens - this way your consistent message may hopefully get through to your dad and her mom, and her. Try to not lecture her as she may stop confiding, but let her know when her behaviour worries you and you are right to let her know you are there for her.
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