Should I tell parents about slutty sister?

Maybe try talking to your sister instead of just telling your mom. This could be a good time for your relationship with your sister to grow into a more adult-like one and less of an ‘I’m telling Mom!’ one. Tell her that it doesn’t make her cool or popular to act slutty. It gets her noticed, but not in a good way. These boys are using her and then laughing at her behind her back and you love her and want her to have more self respect than that. Explain all the real dangers she is facing if she continues this behavior, from getting a bad reputation to STDs and pregnancy. If she doesn’t listen or gets upset with you, leave it be. You tried to help her, but some lessons just need to learned the hard way. Besides, telling your mom isn’t really going to solve anything. She’ll get grounded, have her phone taken away, whatever, but no one can watch her every move at every second and the truth is, people are gonna do what they want to do. They best you can do is to try to make her see that she shouldn’t want to do this kind of thing. I’m the oldest child of three daughters and I’ve gone through every possible sister fiasco, so I understand where you’re coming from. Good luck. ;)