Why don't you wipe both cats with those cat cloths (to be used instead of bathing), to make sure they both smell the same.

Maybe Jupiter has picked up various cat smells at your sisters apartment, which Tinky is picking up on.

Re The Kitten. I agree with Barbara that sorting out your current situation first before bringing a new pet in. As your cats did get on orginally, they should calm down and become friends in the near future.

I would consider taking that kitten to a no-kill shelter, as the current owners don't have the time to look after he/she. A young kitten will be adopted very quickly.

Unless your cats are allowed to calm down, before introducing a new cat, you could end up with a full scale war on your hands. Both me and SASVermount wouldn't wish this type of problem on anyone else.

Of course after your cats make friends, you can always decide to introduce another pet at some later point.