I'm feeling the want to buy a cat in the spite of my sisters disapproval.
Why would you want to go behind your sisters back?
I hate my sister for having pets and she hates cats.
It's wrong to spite your sister just because you hate her pets. It takes a lot of responsibility to take care of pets.
I know what I'm doing and I did this to her before. I'm careful with pets.
You really should respect your sisters wishes. She may have a good reason for not wanting the cat, she may be allergic to it.