What I found helped me most when I was having sleeping troubles was a routine. If you do the same thing before bed each night, your brain will begin to think "oh I'm doing the things I usually do before I sleep, it must be time to sleep now!" and it'll be a lot easier for you to sleep. Your routine could be as simple as brushing your teeth, washing your face and getting into bed. Also, it would help if you went to bed and woke up at around the same time everyday. On school days I make sure I'm in bed at around 10:30 and I wake up at 6:45, and I have no trouble sleeping any more, as opposed to when I went to bed at random times, usually between 11 and 2. On weekends I do allow myself to sleep in a bit more and stay up a little later, but I follow the same pre-bed routine.
If you drink a lot of caffeine during the day, it might prevent you from sleeping. People say that you shouldn't have caffeine after a certain point in the day (I think it's like 3 or 5pm). It's also best to avoid intensive exercise before bed, as it will get your heart pumping and prevent you from sleeping.
But seriously, don't stress about your computer usage. It sounds like I use my computer a whole lot more than you do, and I've experienced no consequences because of my usage, so I don't think you should be worried! If you need anything else or if I've made something unclear, please feel free to message me :) ~Elle
However if it's resulting in you not getting enough sleep, or having to lie in late in the mornings and feel groggy on waking, then you need to find a way of disciplining yourself better when it comes to switching it off at night. Remind yourself of how crap you feel in the morning when you've not slept normally, and tell yourself to switch it off. And if that won't work, as a family member to insist that you turn it off at a certain time each night.
I also wonder why you get up so late every day - don't you have classes, or a job, to go to? Perhaps that's what you need to really be examining - find something requires you to get up at a normal time in the mornings, 7.30 or 8.00, and hence will mean you NEED to go to bed earlier as a result.