You're a contractor not an employee. If your job can be handled off site, on a keyboard, them tell them you will be available off-site. They don't own you as they do an employee. Also go up the chain of command, especially your boss' boss. The further up the exec ladder you go, usually the more understanding you will get when you explain why you need the time.
I agree with this. But , if worst case comes true, an illness could also be landing with your sister and husband. Sure hope you enjoy your time with them.