There was no wrong answer, but your ability to boil it down to one issue is good.
The mother didn't commit adultery in regards to the children, so it is important you change this current situation into one where the children have a good relationship as an adult-child with both parents.
In the immediate, that means you should require all the children to participate in some regular and meaningful contact with their mother. The 15 year old must be made to sit politely with his mother and eat a meal - like every Wednesday night.
The conversation I want to steer you toward is what kind of a relationship do you want to have with the children when they are adults, and what kind of a relationship is appropriate with their mother. It's not necessarily something you leave to the adult-child to figure out - it needs direction.
How will this story be told to your grandchildren?
Volenti non fit injuria