For the record, I agree with Towson, however, that's not what you asked, so...
this site/myself can not be held responsible in any way, shape or form (earthly or otherwise) for anything that may occur if you decide to embark on one or more of the schemes outlined herein. Ok? Good.
1. Meddling with Tea/Coffee: This is very simple. NB: when I say meddling I do not mean poisoning. If you dislike your sister, offer to make her a drink and then either use the teabag to wipe the floor before use, spit in the drink, or wash the cup in loo water. If she really deserves it, do all of the above.
2. Hiding stuff: You may scoff at this but believe me, people are known to be very possessive of their stuff (especially those who write their names all over them). My hint to ensure the full effectiveness of this is to embark on a campaign. Start off small with stationary, then move on to her dolls/teddy bears, eventually moving up to the big items such as her vibrator. If you do this campaign over a number of days, believe me your sister will be a quivering wreck by the end of the week.
3. Sign her up for a dating scheme. You can do this by either placing a classified ad, or if you are able, by signing her up to various Internet dating sites. If you can get into her computer, then you could take it a step further and sign her up to lots of sites which will send her junkmail. This revenge trick gives plenty of room for creativity.
4. Fish. There are many different ways to use fish in revenge. If you can, (and this will need careful planning), you can put a fish into her handbag/sports bag. You do not want to suffer from the smell, but if the fish is placed at the bottom of the bag, and the bag is closed, then the smell will get worse with time. Timing is essential. If her bag is never left unattended, then you could always try the coat pocket.
5. Glue her to her chair. Superglue is a bit excessive, but you can find another type of glue (get some advice from an expert in a glue shop). If you do this, make sure that no one else sits in her chair. Glue could be substituted with toothpaste or even paint.