Husker and Oak, first of all, here is a link to Whistler's paintings:
http://www.jssgallery.org/Other_Artists/Whistler/Whistler's_Mother.htm
But this poem was based upon his life. I can't locate the biography that I wanted to be read here, but it was tragic.
Here's what I remember of it. He was American, but was greatly influenced by the French impressionists. His life was a pretense. He tried to assume the persona of an aristocratic Southerner, when in fact, he was just a plain man, son of a railroad engineer.
When a critic panned his works, he spent all of his money and collections of fine trappings, on a lawsuit, which he won, but which left him penniless. A phyrric victory.
Perhaps with this background, you can better understand the life behind the poem