- Anyone can throw out dramatics and tears to get their way or to manipulate someone into doing what they want.
You understand that, right?
Your MIL isn't sorry in the least. She's screaming and wailing because her (horrible, years long) abusive actions have finally had (very minor) consequences, and now she no longer has an indentured servant to abuse and look down upon.
She thought she'd be able to get away with her mistreament of you and your son forever, and that your husband would continue to place her above his wife and child no matter what.
Turns out she was (maybe) wrong, so now she's playing the sad-helpless-old-lady-victim card, in the hopes that that will trump everything for your husband - the vows he made to protect and honour his wife, his marital harmony, his wife and son's self-respect, dignity and well-being - everything.
...Notice how she only brought out the tears when yelling him into submission didn't work?
Yeah.
Think about it - she couldn't even do you the courtesy of acknowledging your presence in your own home even as she was trying to guilt-trip your husband into 'choosing her over you' and/or manipulate him into having you resume your indentured servitude.
She is abusive, and bigoted, and malignant.
- MIL's failure to adult, and her failure to make others in her household adult, is not your problem:
Firstly, live-in SIL is only not doing her fair share with housework now because they're both so sure they'll have you along in a jiffy to get back to work. How does that not make you angry?
Secondly, you have plenty of BILs and SILs that MIL actually treats with respect, one of whom actually lives with her. Why was she only banging on your door at 8 o'clock at night? Why was she ruining your Sunday with histrionic demands for help with groceries and housework? Why not theirs?
Thirdly, how is it that she is able to get to your house at 8 o'clock at night, but can't go to the grocery store on her own?
Aren't there grocery delivery services where she is? Cleaning services?