I'm going to be complicit in my father's abuse of his wife and stepdaughter
Why would you help your father with his abuse?
That's all I can do for him, his health isn't that good
You should not be complicit in abuse. No matter how much your father means to you, he is committing crimes and needs to be turned in.
I just don't want to lose him.
If you don't want to loose him, then help his wife and stepdaughter get out. This will stop the abuse on both ends.