EnoughOfThis - I honestly shuddered at some of the sentences in your last post, too close to home for me 
Particularly And now I am the one who's controlling (I will come back to that).
STBXH however, has not (and never will) blame his mother, in any way - she is a blessed saint, who gave him 'a loving and secure upbringing' - why then did he and his siblings all bail out on the stroke of 15/16 years of age and move/work at the farthest reaches of the U.K/World from her? In-Laws never took much interest in our DCs, outwith parading them in front of their neighbours occasionally (to uphold the Happy naice Family and Loving Grandparents façade) and they have already attempted to push the eldest (6 years old) into the spare role of family scapegoat (no emoticon for my raw screaming pain regarding that is there?)
Through very confusing, painful, and devastating times, I have become aware that in-laws are Narcissistic to their core - and that my ex was manufacturing (false) difficulties and colluding with them to discredit me (well, the last family scapegoat only went and committed suicide so there was a job vacancy, and I was the prime candidate as I'm not one of them).
Ex is the Lost Child of the family - now completely against me and totally immersed again into their lies and beliefs as he simply cannot accept an alternative to his fantasy family life (his sanity is at stake I know, he cannot, for example, love them but dislike what they do...that is what I'd call normal...instead he has to escalate, collude, ignore evidence/proof, lie, gaslight, me to maintain his mental equilibrium).
The controlling - Ha! Ex says he was controlled by me, another pathetic attempt at excusing HIS inadequacies. Couldn't make a decision to save his life (fear of responsibility/blame) and accused me of "always having a better suggestion anyway" so that's why he (chose not to) didn't make any decisions 
There is a history of sexual abuse in his family, and alcoholism, and of keeping these secrets at all costs (or to the grave for his relative).
It's so so sad but I suppose I need to take whatever steps are needed to protect DCS now and give them a happy and healthy childhood
^ This ^ for me now is my priority too - I only hope I have the strength.