I think Diana was probably talking about Adalind and Sean, and Kelly understood “mom and dad” to mean their mother and Diana’s dad. See this post for my reasons why, based on my own experiences growing up in a blended family.

I also don’t think there’s any reason to believe that Kelly writing in the books means that Nick is dead. I don’t think there’s a hard and fast rule about when a Grimm can start writing in the books. Even if there were, I doubt Nick and his friends/family would follow it. They’re a pretty unconventional lot.

Now, if you mean that writing in the books suggests Kelly has his Grimm powers, which means that Nick must be dead….well, I dunno. It was kind of suggested in the pilot that a family member dying was somehow linked to the next generation getting their powers. But it was never outright stated that this was the case, and subsequent events kind of muddied those waters a LOT.

The net result is that at the end of the show, one of the BIGGEST, most fundamental pieces of the show’s mythology remains unanswered: we never learned what EXACTLY triggers the onset of Grimm powers. All we know is that girls get their powers earlier than boys…but not what actually causes the change at either point. Is it mystical? Is it purely physiological? Is it a bit of both? We just don’t know. And even if we did, we can’t say for certain whether those rules would apply to Kelly, because he’s not just a Grimm…he’s the son of a Grimm and a Hexenbiest. He’s completely unprecedented.

So based on all that uncertainty and statements made by the show producers in several recent interviews, I’m leaning toward Nick still being alive in 20 years. There’s nothing to suggest they want us to think otherwise.