In my copy of the story, the youngest son is named Boots (which, sadly, makes me picture the character from Dora the Explorer. One of the disadvantages to babysitting), but in other versions he's called Cinderlad.
Well, Boots's father's grassfields keep getting eaten on St. John's night by an unknown source. Boots' older brothers try to stay the night and catch the culprit, but are frightened away by earthquakes. Only Boots is brave enough to wait them out, and three years in a row, he finds three grand horses with full sets of armor right by them. One brass, one silver, one gold-much like the three dresses the Grimms' Cinderella requests from her mother's grave. Though the materials of those dresses don't get grander every night (although the last night has her dancing in gold shoes,) Donkeyskin's gowns are colored as the weather, the moon, and the sun (Donkeyskin is often categorized with Cinderella, as the oppressed-maiden-whose-true-worth-is-eventually-recognized).
Now the king of the land has a daughter that sits at the top of a glass mountain with three golden apples. The only one who can win her is the one who can climb the mountain and be given the gold apples. There are three days in which Boots goes out each day, each time with the next nicest horse and armor, and goes partway up the mountain (a little farther each time) and then back. The people are all wondering about this mysterious knight who has found favor to win a golden apple each day yet leaves. Boots' brothers leave him home (sitting in the cinders and ashes) while they go and then tell Boots all about the festivities. Boots feigns ignorance and interest, just like Cinderella.
All the knights of the land present themselves to the king, so he can find the one who has the golden apples. No one does, so he sends notice for all the men to appear before him. This still yeilds apples, so the king asks if there is anyone else. Boots' brothers say there's no need to ask Boots, who has been home in the ashes, but the king insists. (Much like the parents of Cinderella, who insist she cannot be the woman the Prince seeks, until the Prince insists.) Boots, of course, produces the apples and the king and Princess are overjoyed. I like the ending-"and all I can say is, if they haven't let off their merry-making yet, why, they're still at it."
Gail Carson Levine has written a story about "Princess on the Glass Hill." I haven't read it, and I like her novels more than her short stories, but I'm sure it's a great read.