Perhaps the kid put some kind of pill, which he thought to be ecstacy in the drink? Any london twat would rip a 15yr old off... besides he prolly only paid 2 pound for it...
Even more so, there was probably nothing in the water all together... usually, pills are coloured (not always).. but wouldn't the colour have discolored the water?
Perhaps he intended to tell his sister he put E in the drink, to make her freak out?
all these things should be taken into account - he won't get done (well i don't know) if there is no drugs in his mothers blood.. unless of course he actually put rat poison in there instead...
Who knows - god i hate the media