Feeling stuck on how to handle my brother and his relationship

Assalamu alaikum, I really need to get this off my chest. I’m in my early 20s, and my younger brother-just a year apart-has been seeing a non-Muslim girl for a while now, like a few years. I remember noticing how close they seemed, with those long, frequent calls, but I didn’t say anything. Back then, I figured if my parents weren’t stepping in (even though I could tell they were noticing and curious), who was I to butt in? Looking back, I sometimes wonder if things could’ve been different if I’d spoken up sooner, but maybe that’s just me trying to ease my own guilt. I keep bouncing between thinking he’s his own person now, alhamdulillah, grown and independent enough to support himself and make his own choices, and just plain worrying. And honestly, a lot of that worry comes from my mom. She’s been really stressed about him and has no one to talk to except me, which just feeds my anxiety. The tough part for her is that she can’t lean on my dad about this-he completely shuts down whenever it comes up. So she’s left alone with her fears. My dad, even though he’s usually the strict one, has never once brought it up with my brother directly, even though both of them see this as a huge concern. It’s confusing-like, why hasn’t he said anything? Meanwhile, my mom has tried talking to him about it maybe twice, advising him against the relationship. In the end, I know it’s his decision, and I make dua for him always. I did have one talk with him over a year ago, before either of my parents did, because I was curious and a little worried. It was clear he was carrying this secret, even though we all kinda knew. I honestly think if I hadn’t asked, my parents might never have brought it up, despite how strongly they felt. But by then, he and the girl had already built a solid, though rocky, relationship over three years during undergrad. Now it’s been even longer, maybe five plus years. I haven’t really brought it up since, but I know they’re still together. It must have gotten too heavy for him, because recently he came out and told my parents he’s still with her and can’t be without her-apparently, they’ve even tried cutting off contact before. I’m not even sure what I’m asking. I don’t have anyone to talk to about this in real life, and being home for the summer with not much else going on just makes me see my mom and dad’s worry up close. It makes me wonder if I should still do something. Being around him again, I keep feeling like I’m missing a chance to have another conversation, even though at this point I’m not sure any action would help. Part of me worries I’m just ducking a responsibility that’s still mine. I don’t like the idea of approaching him again because I love him and care about him, and I can see he’s struggling with this himself-maybe even feeling guilty or ashamed. But it also hurts to watch my parents, especially my mom, in this situation. And I’m really frustrated with my dad because, out of fear of butting heads with my brother (they clash a lot, and my brother never backs down), he acts like nothing’s happening. That leaves me comforting my mom and second-guessing my own role in all this. On top of that, I don’t know how to act around my brother anymore. Things feel awkward, like we’re both avoiding the big issue. It makes me sad, and I wonder if I should just start asking him about her. But then I’m afraid that by doing that, I’d be showing approval or making their relationship seem normal. And here’s where it gets personal for me: a lot of my sadness comes from seeing how much he texts and calls her, compared to how rarely he reaches out to me or our parents. He never contacts me on his own, and when we do talk, it feels one-sided, with me doing most of the sharing. I’ve joked about it before, but it stings. I want to be close to him, but I can’t tell if we’ve always been this distant or if I’m just feeling possessive now that he has someone special. I’ve tried to think it over, but I’m scared if I bring it up, he’ll think I’m just nitpicking or trying to control his personal life, especially since my parents and I haven’t hidden our disapproval. Any advice or thoughts, or even a wake-up call, would mean a lot. Jazakallah khair.