The sisters are the vowels A, E, I, O and U. The brothers are the consonants.
I am the eldest, and in fact the first of all our siblings.
'A' is the first letter.
My next sister buys alcohol by the bottle and places them on her shelves.
The letter 'E' looks like a bottle adjacent to three shelves.
My next sister always wraps her slim figure in the nicest of evening gowns, which she stitches herself.
'I' is a slim figure. Perhaps the horizontal lines on top and bottom resemble gowns. She stitches herself, because 'I' stands for ego.
In contrast, you can always find my next, more portly sister at the pool hall at a quarter past three every afternoon.
'O' is, in contrast, round and portly. It is shaped like the balls in the pool hall. 'O' is the fifteenth letter of the alphabet, so it can be seen at quarter (15) past three (A, E, I).
My last sister prefers playing blackjack (she sees herself in the table).
'U', shaped like blackjack table and also the 21st letter, an allusion to the fact that blackjack is also called 21.
The sisters appear according to their age in the word 'Facetious'.