A literary analysis of a letter to a little sib:

DEAR LITTLE SIB, <= Overused.
Sorry for being a little slack. <= Who is sorry? You need a noun.
So I am giving you quite a snack. <= Comma before "so".
Not too healthy I know but they're better than bricks. <= Sentence fragment.
Don't eat to many, or I am afraid you will get sick. <= That doesn't rhyme with bricks.
So I give you a word to the wise. <= Colon, not period.
Enjoy your time here cause it really flies. <= Either put an apostrophe before 'cause, or write "because"

Your BIG SIB is in a rhyming mood.<= All caps may mislead readers into thinking BIG SIB is an acronym
You are very lucky he wasn't lude. <= It is spelled "lewd".

Initially, I would classify this as grade C work. Incorrect rhymes and misspellings prevent this piece from earning a higher grade. However, the humble attitude, generous overtones, and worry of "BIB SIB" while writing to his "LITTLE SIB" make this poem a worthwhile read. Yet, deeper meanings manifest themselves upon closer inspection. It is readily apparent that "quite a snack" is analogous to "quite a snake", even down to the similar spellings. This observation causes the previous "humbleness" to vanish, and replaces it with a terrible evil: the BIG SIB is actually going to send his LITTLE SIB a venomous cobra! Obviously the BIG SIB is a hideous and cruel master, forcing his LITTLE SIB to labor away in the brick mines of western China, and then rewarding his labor with death in the form of a cobra's kiss. The third line form the end sarcastically hints at LITTLE SIB's inevitable death, while the final line leaves the poem with the horrifying suggestion that BIG SIB could have done something even crueler. Such a deep and inner meaning definitely deserves a higher grade than a 'C'.

A+, Excellent Work!