SD15 was seemingly doing ok behavior and school wise for a period of time and so on Valentine's weekend, DH let her stay the night at a friend's house. SD10 was at her cousin's house for the weekend as well, so it was nice to have some time alone with DH. SD15 was told that if she and her friends were going to go anywhere, she was too drop us a line to let us know. She didn't, so when she got home we asked her what she did all weekend. She told us that she watched movies with her friend, and that they didn't go anywhere.
Yesterday, I was transferring my laundry from the washer to the dryer and SD15's clothes were in the dryer. The empty basket was sitting right next to the dryer and so I dumped SD15's clothes into the basket so I could put mine in the dryer. Out fell several extremely racy undergarments that I would categorize as not "daily wear" (certain aspects of said garments would make wearing these things impractical). My assumption was that these were either obtained this past weekend, or they were obtained FOR this past weekend, (i.e. she wasn't really at her friend's house or she snuck out with her friend to see boys, or a boy purchased these items for her). We live in a very rural area and it's a trek to get anywhere like the mall, etc. so we would've known if she went somewhere recently.
We confronted her, telling her that if she went to the mall with her friends and bought these things with her own money, we aren't about to inspect her underwear drawer or anything, and that it's her own business. But if she's sneaking around with boys and lying, that's a different story. SD15 then proceeds to say that her older sister (half sister, same mom different dad, not DH's kid) bought them for her and brought them to school. SD15's sister is about 20, and is complete trash. In the past, she's given SD15 alcohol, drugs and cigarettes/vape, exposed her to things that she shouldn't have, and basically served as a terrible example. We told both SD's that they can see their sister under our supervision but not unsupervised and they can talk to her on the phone but because of their sister's lack of responsibility, we cannot trust her. Turns out, SD15 had been using her friend's phone to communicate with her sister and was planning to run away with her. We called SD15's friend's parents and explained the situation and they were not pleased.
My question is... Where do we go from here? We have a 15 year old sneaky brat in the house that doesn't want to be here because she has chores and constantly gets grounded because of HER OWN ACTIONS.