first of all, autopsy?

Were you planning on trying to find a cat to examine your dead cat? Autopsy means "self view" so a member of the same species would need to do the exam. The correct term is necropsy, aka "death view". I'm sorry to be picky, but if I have to read a lot words, my brain starts hurting and that reminds me of when I was in college and on of my bosses at the time harped on that autopsy/necropsy thing.
Secondly, here's what you could do. Take some potatoes, carrots, celery, other shit for a stew and throw kitty in there. When sister gets home, give a hot bowl of kitty stew and when she compliments it, you can tell her that Romeo helped you make it and even inspired the dinner.
CLIFFS:
1. Necropsy instead of autopsy
2. Kitty Stew
3. Sister says YUM!
4. ...
5. PROFIT++
6. sorry about the kitty