Don't feel terrible. You did what you felt was the right thing to do...and your sister is doing what she feels is the right thing to do....and two rights equal a happy cat and a good relationship between sisters. Your sister cares more about Jupiter's happiness than her own cause she's a good-hearted person. I'm happy this has all resolved itself in what's best for Jupiter. Now your sis can adopt that stray and he will bond to her.