You didn’t do anything wrong. Telling you who you can and can’t be friends with, who you can contact, telling you to quit your job…that’s wrong.
You did the right thing by calling it off because if you’d gotten married, the controlling behavior would only have escalated. I speak from experience…I’ve counseled victims of domestic abuse and these are the signs of an emotional (possibly even a physical) abuser.
So now there’s a rift in the family…not to sound trite, but these things happen. You are not the first to be the victim of unfair behavior from family members and you won’t be the last. And if it sounds like I’m unsympathetic, I’m really not. I feel for you. This is horribly unfair.
You’re going to have to grit your teeth, pray to Allah SWT and have lots of patience. Do not lower yourself to their behavior…name-calling, gheebat, etc. Not only will it get you absolutely nowhere, but engaging in tit for tat behavior doesn’t win points with Allah SWT. Be calm, be polite…and do NOT engage with them on this subject. Because it doesn’t appear that they want to be reasoned with.
Live your life. Spend time with your friends, pour your energy into your job and business, read, laugh, pray…live. Again, not to sound trite, but we only get one chance at life. Live it well. If your sister and BILs want to waste it engaging in nasty behavior and causing rifts in the family, that’s their choice…let them waste their time on this nonsense. You know better. 
And above all, remember that this is NOT your fault. You had an option before you, you considered it, and you turned it down because you decided it wasn’t the thing for you. This is not a crime. Your sister and your BILs’ reaction to it is unnecessary and immature.