Just to add my two penneth
.... Your sister obviously and rightly hates animal cruelty but to force a cat to become vegan is in itself cruel and unjust. A cats digestion and metabolism are completely different from that of a dog or a human, which is why it is much more dangerous for a cat to stop eating for a few days than if it were a dog.
I personally beleive that we should not impose our personal/religeous views and beleifs onto our pets.
If it were because of allergies, intolerances, illhealth or such like, I'd be more supportive dietary changes.