How on earth could your WH replace you and your children? No way. He has placed a cardboard cutout in place a stunningly wonderful and witty woman. You should know that narcissist care about presentation, how they "appear" more than how they really are. So he wants to play the part but not put in the work.
soon enough the shine will wear off and they will see the ugly truth.

As far as your brother goes, what an amazing man. How blessed you are to have someone this awesome in your life, how doubly blessed you share his blood. The Chinese have a belief that there is a red thread that connects people who love each other. The thread may stretch, tangle and get worn but it will never break. Ever. I wish I had deeper words but I have never been good at finding them at the right time. He sounds like a warrior.

M 10yrs T 13yrs
BD #1Oct 2015-PA between WAH and COW
BD #2 April 2016-WH resumed PA, she broke it off
Jan 2 2017 WH says he wants divorce
April '17-Letting go
2018 D busted
DD8, DS6, DS3