another vote for reading. always helps me get to sleep. doesnt have to be a boring book as everyone has said, just read a story you want to read. at your age i would recommend the Terry Pratchett juveniles (especially 'the wee free men') and skullduggery pleasant by Derek Landy as well.
if you cant read in the bedroom because of your cousin then get yourself completely ready for bed and in your pyjamas and lie on the sofa reading for a while, preferably with a blanket over you. whatever you do, dont have the telly on. if the parents are watching telly then prehaps you could read in their bed before you go to sleep in your own.
aside from the books (and anything thats really obvious) i do have one other trick up my sleeve, and its called flux:
http://stereopsis.com/flux/
if you install that on your computer, then tell it your location (latitude and longitude can be found on google maps i expect) it will change the warmth of the colours on your monitor when the sun goes down, which will help your eyes think that its night time.
you will need to turn it off if you are gaming on the PC at night, as it will ruin the colours of the game, but if you are just internet browsing then its fantastic. i eventually removed it because i'm a student so i'm practically nocturnal
(meaning i had to turn it on and off whenever i went in and out of games)
however, when it was time for my exams and i needed to be up in the morning i put it back on and it really helped. it made getting to sleep at night a lot easier
