My mother, due to her mother, had the same ideas about what she should be able to say to her children even though grown & get away with.
The problem was is that she raised us to believe that respect worked both ways. So needless to say as I grew up, I couldn't & wouldn't put up with anything that came flying out of my mothers mouth & I let her know it.
After I made this known to her our relationship changed for the better. She then saw me as an adult in my own right. Until your wife makes this distinction, there will be discord.
So whether your wife likes confrontation or not, it is up to HER to take a stand & let her mother know that she is a married adult & that she & YOU are both to be treated with respect by her. If her mother cannot comply with this RESPECTFULLY made request, then it is up to your WIFE to ask her mother to butt the **** out & don't interfere in the 2 of yours business!
So while it might be you that is bearing the brunt of your MIL's attitude, it is really up to your WIFE to settle it once & for all.
After this I think you would be TOTALLY within your right to COMPLETELY STOP any and all aid financial & otherwise that you offered to your ungrateful MIL!