There are several ways around this.
First, ask your Step-sister to please use the Scheduler function on her torrent client. Ask her to set it up so that it limits the download speed when you are using your computer and then downloads at normal speed when your not using the computer.
If she does not comply tell her you will have no choice but to tell your parents that she is being unfair with the use of the internet. Don't
get angry or show emotion to either your parents or Step-sister except for sincerity. You are entitled to your fair share of the internet and if your Step-sister is going to hog it how can you reasonably become well balanced child if there is no fairness at home!
If all else fails cut her off all together. If you can't use it neither should she!
P.S. By the way she is likely to be doing something illegal so your parents won't be too happy about that, as your parents would be liable in any court proceedings, and then that would make her life a misery...
Tell your Step-sister to play fair or lose out altogether!