[Baz]

Is Teddy crate trained? Does he already have his own crate? If so, you now have a way of isolating him from your sister's cat that will protect each from each other, should there be any friction.

This helps if you intend to experiment with seeing if they can cohabitate.

I don't feel anyone can predict how that would turn out. If you are willing to try - it will be a process....and keeping them apart from each other would be an important option for you until you can let them feel each other out.

The only other option would be for you to work with your father to help him realize taking Teddy with you is the only way to ensure his and the cat's safety. On that basis......whether or not he accepts it - internally he would at least understand (hopefully) this is in everyone's best interest.