.
Does she know you're running torrents ..?
'Cause, if not, then the option about a guest account. Create a user account with limited privileges. Let her know the password. Anytime the computer gets rebooted, anytime you log off, she only knows one account to login into. You can set that account to run torrents, on startup/login (you use utorrent, right?).
All around, about as much win as you can salvage. The kid gets to play when you're out, your sis is denied the satisfaction of screwing with you (other than the kid is using your system). Your torrents run under the rights-limited account, which is open and running whenever you're not using it, but at least with fairly minimal security risk.
When you are using it, you login to your main account with the privileges you require for all your other activities.
The real fuck in the ass here is a 3 year old on the system. At all. Any system. I'd seriously punt my sister hard in the twat for that. No child under about 8 should be on any person's main system ... they should be restricted to a secondary system dedicated as a web browser with everything locked down and vastly restricted browser privileges. Only Bad Things will happen to any system a kid under about ten is on.
If she wants to trash a system that badly, she needs to fork the cash out to buy and maintain a system that is about to be scrapped by an animal too clueless to understand it's shitting its own pants.
I feel for you, bro. That girl is too fucking stupid to breed more stupidity into the gene pool. Family. Just can't leave the bodies on the curb anymore.

Usually ...
