She took her in.

Nightmare Moon: “Do you understand now, my faithful subject?”
Nightmare Sunny: “I was skeptical at first, but now I see your point. This is definitely worth it.”

Awww, so cute.

I think it would be an interesting idea that somehow Sunny and Nightmare Moon are somehow related. Just imagine that you’re Sunny and you discover that Nightmare Moon the Mare that tried to cast the world eternal night is your mother or distant relative. I think this would make a good episode especially if the town/the rest of equestrian found out and began to fear her even more because she’s an alicorn. Plus we never really got to see Nightmare Moon that much in G4 and a classic villain would be nice.

Which is which now?

Don’t be intimidated Sunny, it’s probably just Luna in her Nightmare glamor…