I agree Frog...

I don't think that Jacob could fix the things on the island because he didn't know how too... Ben had a tumor not cancer..

I also think that everything that they do is only to serve their own purpose.. They had to keep Juliette there and working so that is why they fixed her sister... If her cancer did ever come back.. Ben could have made up false medical reports to make Juliette think that the cancer had come back and that Rachel would die before she ever got to her to make the deal with her to keep her on the island...

__________________
Please don't give up, Des. Because all we really need to survive is one person who truly loves us. And you have her. I will wait for
you. Always. I love you, Pen

DESMOND: I love you, Penny. I've always loved you. I'm so sorry. I love you!
PENNY: I love you too.
DESMOND: I don't know where I am, but--
PENNY: I'll find you, Des--
DESMOND: --I promise--
PENNY: --no matter what--
DESMOND: --I'll come back to you--
PENNY: --I won't give up--
BOTH: I promise. I love you.