My sister is involved with a non-Muslim man

Assalamu Alaikum, I need some advice about my younger sister. She's 20 and I found out a few months ago she's been talking to a non-Muslim man in a way that's not appropriate. She was in a relationship with him, which is already wrong, but I tried to handle it gently when I first found out. I talked to her privately, hoping she'd understand and stop, because I know how tough heartbreak can be. But two weeks later, she was still in contact with him. I warned her again that it needed to end. Fast forward four months, and nothing changed-they even blocked me on social media so I couldn't see. But I caught her again, and we had a big fight. Now they've unfollowed each other, but I'm worried it's just for show, since she kept it hidden from me before. I don't know what to do now. Should I tell our parents? I can't just take her phone or enforce rules like that. Any advice on how to handle this?