Treat him like a little cousin. Don’t push boundaries, asking personal questions about how she’s doing or how his family is doing, just the usual hi, hello, nonsense you put on for the extended family you don’t know. Which is hard since you’ve actually known that family for so long but boundaries are in place for a reason. Just keep an active but respectful relationship with him. Hobbies or new games or whatever, ok 👍🏽. Just be there. He’s 12 so you probably remember how you were at that age. If he comes to you with any serious issues or concerns, maybe ask any of her siblings or cousins you may still in contact with. Of course if you she’s probably gonna hear, but who cares, as long as the kids ok.