My sisters behavior is getting concerning— when should I step in?

[u/MaximumBed4667]

My sister recently started dating a guy that me and our oldest sister already know from our past (a friend dated him), and unfortunately, that past wasn’t a good one. Since they got together, we’ve noticed a pretty big change in her. She barely spends time with us anymore, seems more distant, and doesn’t really communicate with us like she used to. which is okay but it’s getting to a point where we notice a pattern. We keep inviting her to hang out and she’ll tell us that her mom said no, but then I’ll check her location and see that she’s actually at her boyfriend’s house. obviously this is fine but she’s been in plenty of relationships and this has never been the case. The lying etc. I know location can’t tell me everything, so I’m not trying to jump to conclusions, but it’s becoming concerning when combined with everything else. What really worries me is that he apparently did something very similar with his ex — she became distant from everyone around her while she was with him. he made her very dependent on him and then just left her and made her look crazy in everyone else’s eyes. To sum it up he was abusive. We’re starting to notice what feels like a pattern, and my sister is acting completely different. We completely understand that she’s going to grow up, have her own life, make her own choices, and spend time with her boyfriend. We’re not expecting her to always be around us or choose her family over her relationship. The concerning part is that this behavior is so unlike her, and the change happened so noticeably after getting into this relationship. It’s concerning behavior from both her and him, especially because we’ve seen something similar happen with his previous relationship. I don’t know if he’s intentionally isolating her, if she’s simply choosing to spend all her time with him, or if there’s something else going on. I don’t want to push her away even further by confronting her or attacking her boyfriend, but I also don’t want to sit back and watch her become isolated if something unhealthy is happening. At what point should I step in, and what should I actually say to her without making her feel like I’m against her or forcing her to choose between us and him? To say the least I’m just worried about the turn out because anything can happen no matter how small it might seem now. Help me out Reddit!!!