Anyway, let me summarise this;
Your stbxh is a serial cheat
He had avoided paying child maintenance for his kids, depriving them
In doing so he had ensured that you couldn't continue to live in the family home, and that his children had to leave their home (at least the vast majority of the time)
He takes his kids relatively little.
He moved his latest gf into the family home within 8 weeks, donhax introduced a new woman into their lives - full time, in they're home when they stay there, within weeks
He did nothing to prevent pregnancy with his latest gf and had introduced she's half sibling into your kids lives very quickly.
He's insisting his kids call his latest gf and now baby mama Mummy even though they have a Mum
So basically he's an all round c*nt, bastard, idiot and wanker.
But you're sad he's "given his latest gf a family" ..... A serial.cheat, child maintenance dodger who doesn't see much of his kids, who gets into relationships and parenthood far too quickly, who's irresponsible, selfish, had no morals etc
And she's in an even worse position than you were, because she's not even married to him. And hadn't even had a long term relationship with him.