Firstly, no-one believes you. Sibling blame is the most blatent attempt at a cover up! :P I tried it too when i was young, (when i was 5, not 16 i mean) despite the fact she couldn't even hold a pen at that age :C
As for exam woes, like others say, get busy. You can't control what you dream about, but come results day you'll regret wasting the summer thinking about exams, instead of having a great time, then you'll get swept into AS work work work work.
I admit, i have also had dreams lately, where on results day this girl called Bernice (who talks of nothing but how she is better than everyone with 10A*) beats me by 1 mark on all of my Further Maths exams, and then i got into a fight with Dr Tuck, my Maths teacher, because he said i wasn't deserving enough of his teachings, and thus no longer good enough to play volleyball. It's all very intresting because i've never played volleyball before, and I taught myself FM, and although there is this loving rivalry going on between me and her, i know she's beat! ;D It's only a matter of time. So it's all a bit silly really, but you can only laugh at it :P
Good luck with your exams anyways. Stop worrying! You'll do fine. xxx