I can see in the aftermath of a death, a grandparent stepping up and filling in that role of caretaker of children and helping with the upkeep of the house.
In this case, it seems the mom stepped into that role and is now feeling threatened that there is another woman coming into it and essentially replacing her. I do see her behavior as controlling and unhealthy. And while I might not blame the brother for accepting the help she offered, when she offered it, I do see it as his responsibility to draw the line with her now. I think he should insist that she step back and let him run his own life...to include doing his own laundry, cooking, cleaning, etc.
I don't think we've read anything to indicate the brother is not willing to take control of his life.
He didn't hear his mother order for him so when presented with a drink, he accepted it graciously and didn't make a fuss. He then chose to drink what he had brought when he had a refill. Sounds pretty self sufficient and mature to me.
I think the OP needs to stop allowing her fear of her mother making a scene control her normal actions. If a guest in my home wanted to throw a fit because I confirmed with someon's drink request I'd let them make a fool of themselves and probably enjoy the show while she was doing it.
My advice to the OP is to learn:
"Mom your being silly and overly controlling."
"Mom, I don't want to hear it."