I (26M) found my little sister (16F) and her boyfriend having sex.

First, I'd like to apologize for the weird formatting you may see here. I'm using voice to text on my phone. Anyway, here's my story.<br><br>I (26M) have been an active duty member of the US army for just over eight years. I recently did a 2 year tour overseas and am currently in the process of moving to a new duty station. This past week I went home to visit my family and move the remainder of my items into my new house. I have 2 sisters, one is 2 years older than me and the other is 16 and a sophomore in high school. For the sake of this story, we'll call her K. K is very responsible, got all As in school, has never been in trouble, and has been babysitting and working a part time job since she turned 14. Then this past spring she starting dating a guy from her school, he's on the honor roll as well, plays in the band, and works at the local grocery store. I met him when we got home about a month ago. This past week I had been babysitting my niece full time since we were temporarily staying at my sister's house while we waited to move into our new place. <br><br>On Wednesday morning, my sister and brother in law asked if I'd be willing to pick K up from school and take care of her and my niece for the day, and I said I would, but I'd need to take care of a few things first. Those "few things" turned into an all day event, and I wasn't able to pick up K when I had originally planned. She ended up getting home about an hour before I did, and I figured she was doing her schoolwork. This is where the story takes a turn for the worse. When I walked in, I noticed her car was in the driveway, but she wasn't anywhere to be found. I called out to her a couple of times, and got no answer. When I went upstairs, I found her bedroom door locked, which was unusual for her since our parents weren't home. I asked her what she was doing, and she said she was doing her homework. Again, I thought this was very unusual. She told me she was going to be a while, so I wasn't to disturb her. <br><br>At this point, I knew something was up. K has never been this secretive with me before. I called my mom and asked if it would be okay if I opened the door. She told me to give K some space, and that she'd call her and ask her if it would be okay. When my mom talked to K, K told her that she was just having a bad day and needed to collect her thoughts. I pushed K on it and she ended up telling me that she wanted to be alone when her boyfriend got there. I tried to talk her out of it, but she wouldn't listen. I knew that she was going to have sex with him regardless of what I said, so I told her that if she was going to do it, then make damn sure she talked to her parents first. <br><br>I was wrong, she didn't listen to me. As we were talking, her bf texted her and said that he was there. She snuck out her window and let him in her bf. That's when I decided to tell them that I knew he was there, and that I wasn't going to let them do it. K got very defensive, and started yelling at me. In an attempt to keep the peace, I decided to take my niece to a park that's nearby and get some fresh air. When we got back, K was in the backyard doing her homework, and her bf was no where to be found. I figured he had left, or that I had scared him off, and he never intended on coming over to begin with.<br><br>When we got back inside, I went upstairs to change my clothes and take a nap. When I walked into K's room, I was in for a nasty surprise. I found her window open, the screen missing, and her bed in disarray. I'm sure you can guess what happened next, I was pissed, but more disappointed. I went back downstairs and made some dinner for my niece and I, and when K came down, I had a talk with her. She tried to play it off like she was doing homework for a group project. I asked her why her window screen was missing and she played it off as an accident. I knew she was lying, and I knew what she was doing. I told her that I knew he was there and that I knew what she'd been doing, and that if she wasn't going to be truthful, then she needed to go to her room until our parents got home. <br><br>She went to her room and cried for the rest of the evening, never once admitting to what she'd done. When our parents got home, K denied everything, and told them that I'd been mean to her. They believed her and scolded me for not minding my own business. I tried to explain to them what I'd heard, but they just thought I was being paranoid. The next day I got a call from my mom telling me that K had admitted to having sex with her bf, and that she'd been caught by another family friend who'd been in the neighborhood. <br><br>She'd been caught having sex in the car, and our friend told her parents. My mom and dad both apologized profusely for not believing me. They're both very mad at K, and her parents are going to let her do whatever she wants as punishment. I'm just glad that K was safe while she was having sex, and that she's okay. I called her up last night and talked to her about everything, and she said that she was just scared to tell the truth and that she'd never listen to what I had to say again. <br><br>If you made it this far, thanks for reading.