TBH, this place is as good as any other becarooooo!
Stuff the fact she had an unhappy life, PLENTY of people have unhappy lives, that doesn't entitle them to go around making everyone else miserable.
A phrase or two for you to memorise:
When she makes some shitty observation/comment:
"kindly remind me why any of this is your business?"
I'm sorry, but that is wrong and you are being rude"
"People in glass houses shouldn't throw stoned"
Thanks AWFULLY for your wisdom, I'll let you know when I need more of it"
Call her out - to the point of utter rudeness if you must - every single time. She's no right to be rude/critical to you! none what so ever.
And SAY to your mum "I dunno why you put up with this either, you could buy and sell this one over a million times, your worth a BILLION of her"
Bottom line, she is rude to you, you don't have to see her. tell her that you won't be spoken to like that, and that poor manners have no space in your life - open the door for her and invite her to use it!