Middle school relationships getting too intimate

My little sister and I don't get along at all, she just recently turned 13 and is so immature for her age. She's had so many different boyfriends within the last TWO years and I think it's absolutely ridiculous. I know there's a lot of people who used to do this at my age but I always saw a middle school relationship as "Hey let's sit together at lunch and hold hands walking out to the buses" nothing more than that. But apparently my sister's taken it further. When she was 12, she was on and off with this guy just a year (if even) older than her. My mom would let them hangout outside of school and my sister would constantly lie about going to one friends house, when really she'd go off to his house. I told my mom she was lying but she didn't seem to do anything about it or care. Later on, we found out that my sister had gained a reputation as a "slut," but I'm sure as we all know the term "slut" in middle school is a little different than in high school and college. Anywho, the reason for this was apparently, my sister had started sneaking out in the middle of the night to see him or he'd come sneak over to our house. They'd hangout in the back of my moms soccer mom van... alone... in the dark. The boy she was with at the time then started bragging to his friends about how he'd gotten a blowjob from my sister and how he also got to finger her. There were also screenshots online of him snapchatting her saying "wanna have sex tonight? 😘" and he was shirtless. My sister agreed but fell asleep luckily, and I was able to tell my mom about it all before they got even more intimate. After I told my mom, she spoke with my sister's friend's mom about it and they won't let her friend hangout with the boy because he's also got a bad reputation. From my understanding, they both claimed they were in love and no one understood them. My sister lost so many friends because they didn't like him, and he also lost a lot of his guy friends because they didn't like my sister. What the hell is up with this kind of middle school drama? Maybe I think it's a little much because I'm still on my first boyfriend at 18, but seriously? What do you guys think? Or do you have any experiences of this sort?