@AnywhereAnyoneAnyTime
I think it's possible, maybe not probable, but definitely possible that he has if not an actual second family (ie married with children) then a second 'serious relationship' and possibly a child from it.
Of course, a simpler explanation would be that there's no other woman/other life and that he's simply told his family that he's staying married 'for the kids'. He may have said that OP is a 'crazy bitch' or has cheated on him but that he needs to keep his UK job and UK life as it's the only way to see his children as OP has threatened that he'll 'never see them again' if he leaves her and due to Hague he can't move them to <insert country>. And that his trips 'home' are because he 'needs to get away from her to keep his sanity', and that she 'won't let' him bring the DC for a visit. That'd give his family plenty of reason to hate OP and to be NC with her, wouldn't it? And it would allow him to live a 'bachelor life' in wherever it is they live.
But let's follow the 'new family' thread, shall we? This has been going on for 4 years according to OP. It's possible that in the year or two beforehand he met and 'courted' someone else then in the 1st year of blocking OP either married them or moved in with them. As far as a 'cover story' for only being around every 6 months (Xmas hols & summer hols), he could have come up with any number of cover stories, including working in a 'secret' job for a Govt*, working 'offshore', or in a 3rd world country. But we also don't know that he's not in communication with this other person when he's in the UK. Second phone, communication at odd times, it's all been done before by cheaters.
God knows, there are enough women who appear gullible enough to believe any story that will allow them to 'keep' their current life so this erstwhile 'OW' may swallow his bullshit hook, line, and sinker. We've seen threads right on MN where women seem to believe the most improbable lies in order to not admit to themselves that a man they are married to or 'entangled with' is cheating, gambling, stealing from them, drinking/drugging, and God knows what else. OP herself has at least 'half believed' for these past 4 years that these have been 'simple family visits, only now her mind has 'woken up' and she sees it all too clearly.
It would be pretty easy for this man to lead a double life, especially if his own family is aiding and abetting by keeping his UK family a secret. But I tend to think that he's told them he and OP are divorced and asked them not to communicate with her because, you know, she's 'a crazy bitch'. It sounds as if they didn't like her to begin with, so it probably wouldn't take much persuading on his part to convince them to go NC.
*Had a friend whose BF convinced her he worked 'undercover' for the FBI. She swallowed it even though it really didn't make sense with his lifestyle and the way he behaved. Not to mention it's highly unlikely that a real UC agent would tell a GF that he worked UC in the first place. No surprise when he turned out to be married.