“Do you have any idea what you’ve done Isabel?” said the old man slowly.

The teenage girl sat curled on the hardwood floor, an over-sized white sweater enveloping her hands in sleeves that extended several inches beyond them. Isabel was very pale, her skin was nearly the color of the shirt, but her hair hung loosely over her whole body like a great black mass of waving kelp. It was quite a stark contrast. She glowered back at him from between great fronds of hair, but didn’t say anything.

“Now I’m going to have to re-pour all of them.”

Isabel’s caretaker shuffled back off to his garden feebly clutching a forty ounce bottle of beer. Isabel seemed to snarl at him, or maybe curl her adolescent lip into a semi-sneer.
 

I can't see the unwiedliness yet, of course. My reactions: I'm not hooked. I know that the girl has annoyed the man, but I don't know why, and most of this is spent on describing her appearance, which doesn't interest me. I'd suggest getting straight to the cool thing in the story, whatever that may be.
 

Another thing that grabbed my attention was the the long tag you used, (the old mans said slowly). It made me stop for a second.

Since he seems to be getting onto the girl how about something like : Scolded the old man or the old man scolded.

(LY) words in tags pull me out of the dialogue when I read.

It could just be me though, because I see a lot of published authors do it too.
 

"Isabel! Do you realise what you've done?"

The use of the name would be to grab her attention. Leaving it at the end of the sentence seems to serve no purpose - he knows her name, she (presumably, wild child or no) knows her name, so there's no point to it.

It's not a great deal of a hook - a kid drinking beer left for slug traps. I mean, there's kind of an "eew" factor, but I'm not grabbed.

I like the physical description of Isabel, though. I'd keep that, just not necessarily right up front.