Originally posted by PayItForward
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.
This happens to me every time I take one of the boys to the vet. When we get home the other two hiss and act like they've never seed them before. I just leave them alone and the sort it out usually by the end of the day. Good luck!