They say that an editor doesn’t have to be a writer. I would say that is sort of true to a point. But it really depends on what type of editing you’re performing. For your basic, in depth proofreader/grammar checker I would say, as long as you understand the basics, you can get by.
For doing what I’m doing? As I look over my dad’s book, I try to imagine what he was thinking when he was writing this. What was he trying to convey? With that mindset, I will be looking over what his editor has asked to be revised and respond how I think he’d have wanted to respond.
Luckily, I have known my dad my whole life and therefore I like to think that I knew him pretty well.