feels like my older sister (20f) is losing herself

Salam sisters, this is mostly a vent but any advice is welcome. My older sister started talking to a guy on Instagram in early September - call him F. They clicked right away and since then they've been glued to each other, texting for hours every day. I felt uneasy but tried to give her the benefit of the doubt. Some things about F: - he's 21 and French - he already has a girlfriend - he's an atheist - he lives in Paris (so I’m not worried about her running away) - he knows my sister is Muslim because she told him One day my sister left her phone unattended and, with my worries high, I looked at their messages. I found out she had sent him videos of herself. I couldn’t replay them on Instagram but his filthy replies made it painfully obvious. I was in shock and panic. We were raised Muslim, live in a Muslim country, and never had any guy friends or physical contact with boys. I can’t understand how she went from that to sending nudes to a stranger she met online in just a couple of weeks. I tried to ignore it at first because I didn’t want to confront her, but the next day she took her phone into the bathroom and did it again. I couldn’t hold back - I broke down when she came out and told her everything. I told her to block him and never do it again, but she said, “I’m sorry I won’t do it again but I won’t block him. We’ll just talk as friends now.” They exchanged nudes and dirty messages, and she thinks they can just pretend none of that happened and be “friends.” Am I crazy for thinking that’s impossible? From mid-September until now we’ve had so many fights about F. I cried in front of her many times, but she keeps defending him. She says he’s a good guy and that she trusted him - she even admitted she doesn’t regret sending the videos, only being caught. This is the same sister who prays five times and wears hijab outside. She also seems fine knowing he’s cheating on his girlfriend with her. Mom knows and scolded her, but it didn’t change anything. I can’t keep asking our mother for more help because she already has a lot to deal with, and my sister didn’t seem to listen to her either. It might sound like my sister hates me or our mom, but she says she loves us and regrets hurting us. Still, the next day F is back on her phone and nothing changes. Lots of words, zero real action. She seems like a shell of her former self and I can tell she’s unhappy. What can I do when she refuses to believe me and seems so blinded by this? I know I should pray and make dua for her - and I do, many times - but I’m stunned by how blindly in love she is. This isn’t small: sharing nudes and enabling someone’s cheating is a serious matter, especially as Muslims. I’m really hurt that she put a stranger above her family just because he says sweet things and makes her laugh. I did message F once telling him to stop, and he basically said he didn’t care and they’d keep flirting. Wallahi I’m grateful we’re in different countries. Sorry for the long messy post, I just needed to get this out because I can’t talk to my sister properly about it anymore. May Allah grant you good health and guide my sister. Ameen.