Essie - im so sorry for you, that is truly terrible.
Your mum is just mentally ill, she cant help it, and she doesnt know it. She might have been mentally ill without the GodSquad feeding her illness with off the shelf delusions, who knows, but it cant have helped.
Theres no reason why you will end up the same, you arent in the BibleBashers feeding your mind reality bending crap. You are fully aware of your mental health. If you do end up mentally ill it wont be your mums brand of religious crap trap, it will just be normal common or garden type - forgetting names, switching the oven on then going to feed the pidgeons, wearing gardening boots to the supermarket.
You cant deal with your mum unless you are a fully qualified health expert with drug dispensing powers. Just visiting and listening to her is only ever going to upset you and leave you feeling powerless. I guess the only suggestion that means you can actually DO something is the one to get her certified and put in a mental hospital.
Its like visiting someone with leprosy and having to watch them disintegrate horribly infront of your eyes. Either you stop going to watch the horror, or you find some medication that can help. If there is no medication, I guess the next best thing is for you to spend some time with your dad on his own, give him some relief.
Best wishe with this horrible problem.