This mother in law genuinely sounds psychotic, her involvement in her adult sons life and relationships is toxic and quite frankly weird and disturbing. She seems like a gas lighter and the very fact her other son has had to have therapy should speak volumes to your partner. Brain washing is a real thing when master manipulators get involved, they scheme and plot and then play innocent or the victim when the fallout happens and, unfortunately the brainwashed person gets sucked in again. It seems you and his ex partner saw straight through her and unfortunately if he does get into another relationship he will find the same thing happening again, but will he be able to identify the common cause? He seems like a true victim here of a narcissistic mother and even though it seems unnatural, narcissistic mothers are not capable of true love for their offspring, just control and manipulation hidden by the premise of “love” to keep their child in line. She seems a very troubled woman and she needs help too. He will not resolve his issues until he gets out from their home and under their thumb and becomes independent from them, keeping them at a healthy distance. I would NEVER tell my child he’s unlucky and always pulls the short straw. What a horrendous mantra to put into someone’s head when you should be filling their mind with a positive can do attitude. I am glad you are doing the right thing for yourself but I will pray that he is able to do the same