Hi Dhruv,
Though I am no spiritually sound person nor into tantra mantra but while reading some Spiritual site for solving my problems, I happened to come across mention of your problem and its cause. I think that "might" be your problem and thus hint on how to solve it.
The problem you/your sister is facing could be due to your ancestorial problem. Your ancestors could be waiting for salvation due to less good karmas and thus they trouble their decendants into to get Spiritual gratification. Get this angle checked that your family is not having Pritru dosha. I believe that your family is possesed by some of your ancestor who is troubling your sister(becoz she is the most spiritual/vunerable) amongst all of you. If she gets more spiritual this will help your ancestor shred of his raja-tama gunas and thus come out of Hell/Nether plane. You can read http://spiritualresearchfoundation.o...sixthsense.php for more info. Hope this helped in some way.

Jai Shri Krishna.