He's clearly cowardly and selfish and mal adjusted and irresponsible. Shown by his affair on his long-term wife & mother of his kids, his leaving for his young (visa seeking?) affair partner, knocking her up quickly etc
How is it surprising that he's not bothering his arse to maintain a relationship with his young adult kids?
He's selfish. She probably doesn't want him maintaining a good relationship with his young adult kids ... Because that's attention, time and potential resources away from her and her child.
That's potential inheritance sent their way instead of entirely grabbed by her and her child.
That's the bottom line. She wants him estranged from his kids. She doesn't want him eg feeling obliged to help them financially... A natural result of him having a good relationship with them. She wants any bonds that might lead to resources of his going to them, broken. She wants her name and her kid's name on any will of his, noone else's.
He ... Goes along with that, for a quiet life.
He doesn't care. He's too foolish and low integrity to think about it or care or do the right thing and treat his kids equally.
How you stuck such a limp lettuce and selfish idiot for so long, op, I don't know.