In Islam, your sister has the RIGHT TO SAY NO! Your parents have no right to coerce her into a marriage she doesn't want.
Whether its forced or coerced into marriage or emotional blackmail........where by she has no say, the marriage if it occurs is completely void under Islam. And you should really speak to your parents because what they are doing in Islam is considered a sin, and I would say quite a big sin in fact. Everyday she has to be someone she doesn't want under a void marriage in Islam, is more and more sins for your parents.
I honestly hate when I see these so called "Muslims" doing this, best way to get support is to talk to someone else in your family you trust and knows their Islam very well to counter their cultural values. Its not easy, I'm still struggling to get rid of some of the stupid cultural and traditional ideas in my family, but so far things are getting better.
Which you can do, if it doesn't work then talk to your local imam and get him involved........if your parents are those cultural south asians, they'll succumb to how the community sees them, thats what they care about, the way others perceive them
edit: just to clarify that is not a arranged marriage, but a forced marriage where the girl hasn't given permission for such a marriage..........arranged marriage is where the person asks for family, etc to look for them someone