Mycroft is the eldest brother, and his burden is his worry.
Sherlock is the middle brother, and his burden is his resentment.
Quentin is the youngest brother, and his burden is his distraction.
Mycroft is seven when Sherlock is born, young enough to be fascinated by the tiny living thing that is his brother and old enough to take some measure of responsibility for him. He's twelve when Quentin follows, and still too preoccupied with his first brother to pay the second much heed.
Sherlock is twelve when puberty robs him of his good nature and he falls out of love with Mycroft's overbearing style of impromptu childrearing. He's almost twenty-five when he notices Quentin for the first time in his adult life, and when that finally happens he has no particular idea what to make of him.
Quentin is ten by the time he can reliably escape his mother and his brothers to find himself in his solitude.
(He finds himself, ironically, in that solitude of crowds called the Internet, in technical articles written by distant geniuses and how-to guides posted by self-satisfied adolescents with more time than ambition. Quentin surpasses these informal tutors within months of study and comes to look down on them with no small amount of smugness.)
(Sherlock, at this point, has already found himself in stimulants. Mycroft still has yet to follow his brothers' lead in this respect, but he's found plenty of other people and pried them open as if his answer will be inside, so far without finding anything but beating hearts and dissatisfaction.)
Their burdens are their relationships with each other.
Mycroft's worry leads to his control, and he exerts it professionally, with an iron fist covered in a kid glove. He doles out jobs he think his brothers will enjoy and do well, and they accept them either with a shrug (Quentin) or much sneering (Sherlock).
Sherlock's resentment makes him ambivalent, because there's no game in resenting people one never sees and no fun in seeing people one resents. He compromises with moaning and dramatics and flouncing, which his brothers either tolerate (Mycroft) or ignore (Quentin).
Quentin's distraction is his burden and his vice and his greatest joy all in one; no sane psychologist would ever diagnose him with ADD, but there's something of the disease in how, undirected, he flits from project to project, person to person, never leaving anything undone per se but never seeming to derive anything lasting. Wanting to love his brothers had been a phase and he is well over it; now that they are adults and Mummy is dead, he doesn't see them regularly. Sometimes he forgets they exist, and he doesn't really know or care how they would take that.
(This is a practiced apathy born of necessity and sentiment. It's professional and very becoming on Quentin, who is by now more often known as Q. It's an amusing coincidence, assuming Mycroft didn't do it intentionally, as a subtle and incomprehensible jab at his attempts to distance himself from his brothers, which is to say his birth.)
(His birth is important, but not that way.)
