I’m guessing because his wife was always overseas like his son said, he was probably tired of it and they were already separated. Then he met Mang Mang’s mother, fell in love and decided it was time to officially get a divorce. Some couples only separate instead of divorcing (in most cases date other people), incase it maybe a phase or something that can be worked out, until they decide to divorce or stay together. So, I don’t really consider her as other woman (yet).
Yes, they’re step siblings not half like him and Cong Cong. So they’re not related by blood, not incest. They met when they were older and apparently didn’t see each other for years until university. So they’re more like childhood friends rather than step siblings from what I’ve gathered so far.