Why don't you sit down with your mum and talk though your concerns with her ? Tell her you need time to accept this man, your mum will not expect you to love him straight away, just try to get along with him for your mums sake.

Don't forget he will never replace your dad (I'm sure he knows that) but if you are lucky he might in time become your friend.

I think that Mozart picks up on your feeling for this man, he knows you don't like the man and he reacts to your emotions. If you decided this man was alright, Mozart would calm down too.

I would try and make an effort esp with his daughter, as she is in the same suitation as you are. She will be just as concerned about her cats, as you are about your dog.

This means you can both work together to make the animal introductions easier, so no animal has to leave. Mozart might hate strange cats but I'm sure he can be retrained to leave these cats alone. Maybe having a step-sister might be fun ? You'll certainly have a lot of things in common, including you are both animal lovers.

Pam and Christian's Mommy both have mixed cat/dog households. Try PM them for advice on the introductions.

All the best, I hope things work out for everyone