The speaker was the oldest of the three brothers, so he figures it's his job to keep everyone's spirits up. He does his best to keep his two brothers cheerful.
The other two brothers did as well as they could, too, given their different personalities and the circumstances.
The youngest brother was their father's favorite, because he looked so much like their mother. He has her forehead and her bright blue eyes.
The speaker says he felt really sorry for his youngest brother – he was so young and sweet, it seemed especially wrong for him to be in a dungeon like this.
The youngest brother was a good-looking kid with a really sweet personality.
But the speaker distracts himself with a parenthetical, side comment: he compares his brother's beauty to "day," but then reminds us that "day" isn't exactly beautiful to him anymore, since he's been in prison.
But then he gets back on track: he says that his brother's beauty was like a day up in the Arctic, where you don't see a sunset for the entire summer.
So the youngest brother is associated with summer, and daylight, and freedom. His natural personality is to be cheerful.
The youngest brother only ever cried when someone else was hurt or unhappy – he never cried for himself.
When he was sympathizing with someone else, though, he could really turn on the waterworks – unless, of course, he was able to do something for the person, and "assuage" their pain.