Why is everyone complaining about this, dad wants some fun with his wife not a big problem and all of them are kind of fine, if they all yearned for each other then they could meet in like 30min to an hour since they don't live across the country, i as an example didn't knew my dad got married again until he casual dropped it in a conversation 2 years later, i knew he was together with my step mom for like 6 years but just not that he was married.
Yeeez, yeah things can happen but it was not in malice like if his dad would convince his mum to move away without saying anything would be a real dick move, not just hiding that he has a sister, mean = yeah, asshole/evil = not really.
Some people die internally without social contact especially family, others don't give a fuck.
There are different people in this world, and i for one can understand the people in the manga not needing an all touchy cuddly feely social contact.