I think it would be more like a twin or clone of Juliette being the mother, as it was still Adalind who had taken on the physical form of Juliette.

This post leads to a more general question: does magically transforming into someone else, also mean having the same DNA as that person? Or is it a spell and everyone sees and believes what is only in the mind?

And if it's Juliette's DNA, are Kelly and Diana still half siblings?

Shoot. Let me put on my thinking cap. I would say they're not siblings if it's Juliette's DNA. I like the clone aspect.

Last night at work I had a brain fart. Do you remember that Juliette turned into Adalind in order to make Nick a Grimm again? After the spell wore off Juliette became a Hexenbiest. I suppose part of Adalind rubbed off on her. So there's a possibility that Juliette could be Kelly's mother. Or...Juliette and Adalind could both be the mother.