My cat is a big neutered male and my sister's cat is a small female. Both cats are three years old. When my sister is away on holidays I have to take care of both cats. Unfortunately, they hiss at each other, are never relaxed, sometimes my cat attacks the smaller one, and though there's no serious damage done (after some meowing he stops), I'm afraid to leave them alone.
This is going on for days and I don't see any considerable progress.
How can I get the two cats to coexist in peace?
Will they eventually learn to tolerate each other, or some cats are just "incompatible"?
I will be minding my sister's cat for about one more week, then they likely won't meet for another year.