My (20M) Younger Sister (15F) is Acting Out of Character and I Genuinely Don't Know What to Do

Hi Reddit. Longtime lurker, first time actually posting, so apologies if this is a bit rambly. My apologies if the English isn’t the best. I’m 20M, at uni, graduating soon and have a lot on my plate. But this situation has gotten really out of hand and I don’t know what to do to fix it. 

My (20M) younger sister (15F) was dating a boy from her school (15M) for the better part of the past year. It was her first proper relationship. After a school break, he started getting distant and ignoring her quite a lot. She would literally sit by her phone for hours waiting for him to text. She wouldn’t go to sleep until he did because she was scared he’d break up with her while she was asleep (and he knew that).

They fought fairly often. From what we found out, whenever she tried to talk about her feelings or wanted reassurance, it would turn into an argument. Then about a week ago, he broke up with her over text while she was asleep. Long paragraph about not feeling happy anymore and all that. She woke up to it and was absolutely inconsolable for hours. My mum and I stayed up until 3am trying to distract her and cheer her up.

Later, we found out that the same day he broke up with her, he had already messaged saying he’d “made a mistake.” They had basically been talking normally the entire time she was crying all weekend. She eventually admitted she wasn’t even crying because she was heartbroken - she was crying because she thought we wouldn’t let her get back with him. (which, obviously)

When he suggested getting back together, he said they should do it in secret. No timeline for when it wouldn’t be secret anymore. He’s gotten mad at her before for talking about their relationship with friends, so this was probably why that idea was tossed around. My mum and I don’t really trust the whole secrecy thing.

Some of the messages we saw when we checked her phone seemed sexual and just… off. She’s gen Alpha and they have a weird way of saying things, so I’m not sure if I interpreted them correctly, but it was definitely really weird.

My mum and I confronted her about why she would want to get back with him. It escalated into a fight, during which my sister locked herself in the bathroom for 20 minutes. When we got her out and my mum tried to confiscate her phone, she screamed at the top of her lungs and dug her nails into my mum’s arm hard enough to leave marks. She’s never reacted like that before. She then put on her shoes and threatened to run away, and when asked where she’d go she basically implied she’d go to him. In the heat of the moment my mum said “him or us.” My sister responded “him.” When asked why, she said “because he makes me happy.”

She keeps saying we hate her because we won’t support this relationship. The thing is, she’s generally been treated quite softly her whole life. She’s the youngest, rarely punished, and tends to get her way. We’re not exactly well off, but she always gets the things she wants. She’s aware of this.

She also has a history of lying, mostly to avoid consequences or for attention. She’s almost caused financial problems for us before because she lied about something simple that just needed an apology. She doesn’t have many close friends either, which makes this feel worse.

She’s never reacted this extremely before. That’s what’s scaring me the most. We’re genuinely worried that if he breaks up with her again or if we force this to end, she could spiral more. Right now she’s grounded, but she sees him at school every day so it’s not like she’s not seeing him.

I know they’re 15 and teenage relationships are dramatic, but the secret relationship thing, the isolation, how dependent she seems, and how aggressive she got feels bigger than just normal teen drama.I don’t know what the right move is here. I’m not sure if he’s been telling her things or if she’s fallen into some weird places on TikTok. Recently overall she’s been changing her attitude - she’s been saying some weird things to my mum and acting like she’s been treated like Cinderella all her life. She’s been taking a psych class so I think that may be where she’s getting these ideas, but I’m in the psych field and I’ve been trying to nip it in the bud (with no avail). Help???