I think my little Sister was groomed by my Ex best friend.

Okay so a little backstory, i was friends with this guy for 10 years, we went to the same middle school and he had a crush on me, after he confessed one day and i turned him down and we had no contact for 1 year until he wrote me again.

My sister is currently 17, she's been a big maneater since she's been like 14.
It always gave me worries because she would just stay out late, text random guys from instagram and meet with them, my parents didnt say anything and never do.

About 2 years ago it was really bad, she was dealing with 1 stalker and a few other guys who kept showing up at our door because she would have her snapchat map on for everbody.

I am diagnosed with Mild depression, have identity issues and anxiety, so i am a little "quirky", as people say, i dont function like the rest of my family does. So my sister has been sometimes a real Biatch to me, yelling at me and talking down to me like im a little kid or beneath her. I would talk to my Best friend about it(Now 21M). And he'd mostly be reassuring, but two years ago there was an incident where our front door was posted on tiktok by a guy who'm my sister had broken his heart. And i FLIPPED, i instantly texted my best friend what the fuck this is and what not and he said he would talk to her (At this point they had already conversated and my sister would snort at me but listen to him..so i thought maybe this way she will listen), and after he has texted with her he texted me again and before he said anything he went "But dont get on her ass about it, its not like he actually doxxed you guys", wich...i found to be a stupid statement and we had a fight.

A while later they would become awfully close, and i didnt like that, i infact hated it, he was my best friends and now he was all buddy buddy with my little sister who most of the time treated me like garbage? I felt betrayed HARD. There was a lot of big fights and i would keep bringing up the fact its weird that theyre so close and he considers her a really close friend when they have a big age gap, he would keep saying "Its not weird, theres no age with friends", and even after a big fight he talked like me and my sister are property, saying stuff like "I dont care, im done hearing about it, you me and her are gonna be friend, no matter if you like it or not" . My sister would get on my ass about it too. Even once christmas party it got dropped and only my aunt and cousin was on my side. Not Even my own mother defended my feelings, even when my aunt brought up a situation in her past where it was the same with them, and my mom reacted the same way i did, and she STILL wouldnt understand it.

After some big betrayals i decided i needed a break, and i told him i would take a break from texting and interacting because i couldnt deal with that betrayal and the constant bickering from this too. During that time they got closer and closer i guess, calling EVERYDAY for LONG hours, them cuddling, Sleeping in the same bed, hanging out with him at his place, rubbing her legs. And mind you she was 15-16 during that time and he was 20.

I kept my distance for a long while now, not involving myself in her business even tho she keeps bugging in my buisness and wanting to judge MY relationships. But yesterday i saw a repost pop up from him, about a gf, and i was confused, because my sister also has been reposting bf stuff but when i had asked she always gives the same response "I just find it a cute couple" "Its just a repost, nothing meant behind it". And i had a bad gut feeling, She was out again yesterday until 23:30, Central Europe Time. And it was with him, alone. After the repost i had a bad gut feeling and i waited by our window and looked outside cuz theres a bus stop there.

The bus came, they got out, went up to our door and i just see him lean in and kiss her...

My stomach instantly dropped, making me want to puke, i had said before if he ever tried anything with my baby sister id have him put under the earth. And there was my proof.

I even told my dad about it, and he didnt give a single ounce of shit for it.

I dont know if im gonna ask my mom about it cuz i dont want a fight.

All my other friends say that is predatory asf and my parents dont give a shit about my sister.

I don't know what to do, because i wanna call him out and scream at him for being so disgusting but i think that will just push my sister further to him. Things ive told her in trust she has told him before so he's pitted my sister against me im sure.

She turned 17 this year...he's turning 22. There is also another allegation standing in the room about him but its not proven so i wont mention it.

It just makes me sick to my stomach thinking about what he mightve done to her already, all i ever did was try and protect her and suddenly he comes in and prosumably, groomed her cuz he didnt have any luck so far with gf's his own age and thats supposed to be it?

I feel so helpless, and i still live with my parents so if she brings him over i cant throw him out even if i want to. I just wanna know if there is any chance of saving my sister or with a heavy heart just need to remove myself from the situation since no one wants to listen.