Guin - first of all, let me say that I like reading your posts. They’re generally intelligent and thoughtful, and usually have a lot to add to whatever threads they’re in. Whatever RL may be like for you, you seem to have your head pretty well together here.
I’ll add my voice to the chorus of those who are urging you to move out if you can. The gap between 16 and 23 is a huge one, and she’ll be easier to take in small doses. Not to mention, it’ll be easier, once she’s grown up somewhat, for the two of you to become friends if you’ve just plain given things a rest for awhile.
There may be perfectly good reasons why you can’t move out just yet. If that’s the case, then you’ve got a tougher row to hoe, and there’s only so much you can do to make things better. But let’s add 'em up:
I’ll modify *Barbarian’s suggestion about where the computer goes - your mother’s room may not be feasible. But if there’s any ‘common room’ - living room, den, rec room, even possibly the kitchen if there’s a safe corner away from food prep/eating areas - besides where the TV is, see if your mom’s amenable to letting you move the computer there. That would at least deprive little sis of any excuse to come into your room.
Second, I’m gonna build on iampunha’s suggestion about escaping. What’s your schedule like, and what’s your sister’s schedule like? (I expect the latter will change in a few weeks, with school and all.) When is she usually around, and when is she usually at work, or off with her friends, or whatever?
If there’s any semi-consistency to the times she’s home/away, try to use that to your advantage. Do the things you have to do (or enjoy doing) at home at the times she’s normally away. And escape to the library (libraries have computers, as well as books), a local park, or whatever at the times she’s normally home.
Of course, if she’s always around and awake during the 9pm-midnight timeframe, there isn’t a whole lot you can do in the way of escape, other than change your sleep schedule and become an early-bird. (Tough to do, but it might be worth it: it might be easier to be asleep while little sis is home, than to put up with her. And you’ll have an extremely legitimate beef about sis to present to your mom if sis plays her music loud when you’re asleep, rather than when you’re just hanging out at home.)
But do your best to minimize the times and means of contact/potential conflict with sis. Maybe you and your mom can brainstorm together about how best to do this - it’ll convey the message that you’re trying to minimize the stress. And you can suggest things like “if I change my schedule so I’m in bed at 10pm, could you lay down the law if she starts blaring her stereo after that hour?” which has the chance, at least, of getting your mom on record, in advance, about ways she’ll help you out and take your side in this.
It’s gonna be tough, but I hope this helps. Good luck!