Embarrass your sister about their crush - works on brothers too.
Find out who your sisters crush is, them use it against her in heaps of ways. You could start by making embarrassing posters using the keep calm poster generator available at www.keepcalm-o-matic.co.uk ,with a message like (insert your sisters name) and (insert their crushes name) sitting in a tree, K-I-S-S-I-N-G. Make sure when doing his, if using this website, as it will be going online (when you generate it is public for anyone to view) you use a fake name or anonymous. Also select a background from the 'love' category and a font like 'always in my heart' or 'dancing script' or 'chewy'.
You could then post this to other social networking sites (I have never personally done this) or print them and stick them in her room, put them in her school bag, or in her locker (if your school has lockers, and you attend the same school...). You could also show your friends and email them to her. I have tried this and my sister and was definitely very embarrassed (especially when I showed my best friend!...). The only risk is if they know your crush or another secret and use it against you, otherwise it should work out perfectly!! ;)
NOTE: you don't have to make poster in keep calm generator, but this is what I used. Any other websites or software should work fine, but options are endless and therefore I will not be explaining how for EVERY one if them ;)
NOTE: I was going to attach an example, but it wasn't the right format so I will attach a link to the example I created instead!! Here is the link: