I don't like this mil business one bit. She's not adding to your lives at at, and she's effectively grooming the dc against you I suspect.
She is not supporting them, she is not providing a healthy emotional environment for them, never has done.
I sincerely advise that you scale back contact to a more ad hoc basis. Otherwise if a solid routine is established, she could take you to court for access or stop you making bigger decisions about where you live etc.
A person with class would not reply with the message she sent, they'd have feigned surprise and even gone as far as to wish you all the best. Nothing more.
That she is having you watched is unacceptable.
You must scale back contact now, you must move well away from her and her cronies.
You need a clean break and a proper fresh start.
Ultimately it doesn't matter what anyone thinks, but you have a right to a private life and not feel watched all the time. Her comment will mean that you'll never relax where you are now. Nobody, least of all you, deserves this level of judgement.
You're the one that's here. You are the one who stayed. You're the only parent the kids have, and a bit of fucking respect wouldn't go amiss. Her son let you all down somehow, hard fact I know, but it's the truth. she could choose to make your life and the kids lives better, but no... Far more important to her to dominate, intimidate and ridicule. No wonder her poor son had no chance of procssing feelings and saving himself.
Get your family away from her and her clutches, she'll bring you all down too.