Ignore the beast! LOL. I think husbands sisters can b like this all over the world, not just restricted to desi's. bcos my one of my english friend has had a similar problem with hers. She was plan mean, they lived near eachother. However, as soon as friend moved away very far, SIL was amazingly all sweetness and light on her next visit.
Moral of the story - if she is so much a thorn in your backside, MOVE FAAAR FAR AWAY.
[staying away is the bestest solution for this problem honey, i'm suffering from almost the same problem.. and here my SIL is elder than me - so just imagine it's more worse here.. she's 28 years old unmarried nand of mine.. and this is the best policy to just ignore them "forever" and give a deaf ear to whatever they say to you, lol - cheers.]