Sara, I know what I've told you before, but, after thinking about it more, I say - Do what your heart tells you.
I am sure, your sister is a great person and she will take good care of Jupiter or the other kitten if she decides to take him. Talk to her.
I know how complicated it would be, if we had to separate with my sister and brother. Who will get the cat???? I wouldn't want to give my Juni, but still she's not only mine