Sara,
Take a few steps back, and think about things for a moment. The first concern ***should be**** from the kitties perspectives- all THREE of them. I think that is the way you approach it with your sister. No question Jupiter will have a great home, either with you or your sister. So, think what would make Jupi happiest...and that is probably with Tinker. THEN, let your sister know how you are thinking/feeling - for poop's sake- she is your SISTER- not some stranger. So, lay it on the line, honest, upfront, and from the kitties welfare. Let her know that there is a KITTEN that needs a loving home...and maybe she could see her way to adopt him/her....to make a better life for all three kitties.

As to your BF(or husband? I can't recall) 'wanting' the kitten...well, that is only confusing the situation. That is the second concern, once Jupi's home issue is worked out. THEN, the kitten's home can be resolved. Same thing with the kitten...either you or your sister would make a great cat mom...so, think of it from the kitten's perspective, first.

I agree with Jen, let your sister read this, and see where you are coming from. End result, this will all work out. Better two people stepping up to the plate to adopt a kitten...then, none, as is all too often the way.