My sister has a habit of getting into bad relationships so I’m not sure if I’m judging too quickly or basing it off her past, but I feel like her latest partner is controlling and manipulative and I’m really worried about the impact it’s going to have on her children.
They got together in February and it was instantly so intense - all over Facebook about how much they love each other. He kept posting things like “she’s mine” with pictures of her, constantly posting on social media about how much they love each other, despite being sat at home beside each other, and they even posted a ‘one week anniversary’ photo collage of all their happy memories 
My sister almost instantly moved herself and her two young children into his house, and that’s where they’ve been since lockdown. He now refers to himself as their dad. Posts pictures of them on Facebook referring to them as ‘our son and daughter’. He even got a tattoo of their names. We’ve tried to tell her this isn’t normal, but she’s totally smitten and won’t listen. It feels like he’s trying to distance her from her family by flattering her and telling her we’re all just out to get her.
So the latest thing was last week, it was my daughters birthday. She shared a picture of her on Facebook and said ‘Happy birthday to my Ella (name changed obviously!) and he commented “I think you’ll find she’s ours now”. This has really got to me. He’s met my child twice ever, he’s not her uncle, I don’t know him and it’s weird him being so possessive. It just made me feel really uncomfortable but I can’t quite put my finger on why. I want to say something to my sister about it but I know she’s going to get defensive and say he was just being cute.
Am I right to feel so uncomfortable about this man? I’m just seeing so many red flags but she’s so defensive of any of us try to bring it up.