I'm so sorry to hear about your sister. There is nothing one can say to ease the fear, worry and heartache that goes along with watching those with cancer suffering or in pain.
I moved to take care of my mother for the last 6 months of her life, but if I had to...I would have taken in to my own home.
I did what I did because I loved my mom. You love your sister and just want what's best.
You do whatever you feel will be the easiest and best thing for YOU. If taking your sister in and caring for her is what you want to do, then go ahead and do it.
Your father needs to learn that he can't always get his way and he should grow up enough to put everything aside and come see your sister.
I sincerely hope that things work out for you and your sister. I know it has to be tough, but hang in there.
BJ