What a tough situation.
I recommend doing a preliminary search with everything you know just to see if you can find anything concrete. You can sign up for free trials on ancestry.com and myheritage. Both sites have a huge database of public records. Depending on what state he was born in, you may be able to find the birth record or his parents' marriage license.
Good luck.
We've been down this path. No one has been able to locate anything. It's almost like he doesn't exist, and his parents didn't exist.
If this situation was easily solvable by just google fu I think someone would have cracked the case by now. I got all the information I could, and I can't find an online presence, or even any indication any of these people ever existed. That's what seems so fishy, because even with the small amount of limited information that's been given you should easily be able to find all kinds of records to verify it, which would lead to much more specific info. But it's a dead end right out of the gate. No record of his birth, no record of his family's death, or even their existence. No marriage licenses. No property records.
If this was his real name, and the names of his family, I would think you'd find SOMETHING to at least indicate they actually existed. But nothing. Which is making me thinking it's potentially fabricated. If his name, and his family name, and the DOB, and circumstances around his family are all just fabricated, then of course any searches are going to lead to dead ends because none of it actually exists. It's a wild goose chase.
My sister stopped responding to me when I suggested she sign up for credit karma and check all the accounts in her name just to make sure everything is on the up and up. I tried to frame it not like I think he is doing something...but it's a good idea overall to keep tabs on that stuff. But no response since then. I know she's getting communications though because we've been talking in the family group chat about our parents. My mom is in the hospital after suffering a stroke, and my dad, who was already starting to lose his mind IMO, has been bonkers and I think this whole stroke event has pushed him a bit further over the edge, and a TON of family drama has ensued in the chaos. I also questioned her about the 23andme results, but she obviously dropped the issue and isn't going to pursue it.
So I don't know if she's aware of his actual past and is in on the cover up, or if she's just in denial about it and refusing to pursue answers. Neither of my parents are really approachable about the subject right now.
The alarm bells are still going off inside my head, but I don't know how to approach it without cooperation from my family. I like the guy, and he seems nice, and all of his actions seem to indicate he is genuine. He's deep into the family and putting up with shit from them, and it doesn't seem to make any sense to me unless he was genuinely in love with my sister. I can't figure out what the grift is if there is indeed a grift. But also nothing about it is adding up to me and it's making me uneasy because I don't understand it.