Your situation reminds me of that butterfly analogy....if he was truely yours, he'd come back to you.

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.
I can't even tell you how happy this makes me. When I read this whole thing, I was very upset for Jupiter. I'm glad he's coming back to his rightful home, with his Tinky and his meowmie.

You're sister will be fine. There are a million wonderful cats out there to give a good home.

YAY!!!!!!!!!!