The band’s instruments consist of a set of drums, several guitars, a lute, a cello, a bass guitar, and some bagpipes. They were “all extremely hairy and dressed in black robes that had been artfully ripped and torn” when Harry saw them at at the Yule Ball (GF23).
T-shirts for the band are bright purple with the words “The Weird Sisters” emblazoned on the front (OP38).
"The weird sisters" is a term from Shakespeare's Macbeth, referring to the three witches who accost Macbeth and foretell the future when they hail him as "king hereafter". These characters in Macbeth recite the incantation "Double, double, toil and trouble" which provided the lyrics for the song used in PA/f (see Hogwarts choir).
In Norse mythology, there are three sister-goddesses of fate - the Norns - who are also referred to as the Wyrd Sisters. The archaic term "wyrd" means "fate" or "destiny".
An all-male band called "The Weird Sisters" is likely a two-pronged joke: First, it likely references the Canadian rock band called the Barenaked Ladies (which was very popular in the late 1990s and which consists of fully-clothed men). Second, upon first encountering the Weird Sisters in Macbeth, the character Banquo expresses confusion over their gender and comments that they have facial hair (indeed, they would have been played by men - and probably bearded men - when the play was first produced). --BB
From the Web
Harry Potter Wiki: Weird Sisters
WizardingWorld (Pottermore) features: The importance of music in the wizarding world