Anxiety over behaviour of partner's sister

Hi Background - I had some on-going problems with my partner's sister last year. I found her behaviour a little unusual towards her brother, and in turn, to me. Some examples - very touchy feely with him, stroking his arm when we were out with them and their mum, constantly messaging him telling him she missed him when he moved in with me, causing arguments between myself and him by trying to stir things when he was with her and his mum on his own, private messaged him after I posted a photo of him in a suit saying 'you look so cute in that new pic'. Basically just things I found strange from one adult sibling to another and something I'd never seen or heard of before amongst friends or in my own circle. I tried to talk to him about this and finding her behaviour somewhat odd and unusual but it resulted in a big fight and us almost breaking up as he was angry that I found her behaviour weird. He told her about it and made me apologise to her... for the sake of our relationship I did. Since then she has caused more arguments between us and another more serious fight over her repeating things to him from a private conversation I'd had with his mum in confidence who told me I could talk to her. I have since obviously realised I can't trust her either. That's the background. Today I'm sorting through a pile of usbs we have knocking around, I discover all are mine bar 1 as when I put it in I see things that aren't mine including a video named a song title that I remembered my before playing for me on one of our first dates. He introduced it to me and told me he first heard it when his sister used it in a video she did for school/media studies. So I click it to check out this short film... It's him and her pretending to be a couple, texting each other, getting ready to go on a date, and pretending to kiss at the end, with her doing the voiceover throughout. It's from 5 years ago, she's 16 in it he's 24. I've felt sick and anxious all day after watching it. Am I being silly and irrational to find it weird? He's asked me what's wrong and I've said I can't talk to him about it. After how bad last year was with her I don't want a repeat of it but I don't know how to process this other than keeping telling myself it was 5 years ago, before we met, leave it, it's not worth the argument...