I can definitely imagine Diana wanting Juliette and Nick to be together because maybe then her parents will be together, especially earlier on.
I don’t think she fully accepted it until “Blind Love,” tbh. When she tells her father, “I guess we all love each other,” I think that was her coming to understand and accept that her mother and father aren’t going to be together, and that that’s okay, because they both love her and she loves them, and Nick loves her brother who she also loves…and that’s enough for her.
One of the coolest things about her character is how quietly inquisitive she is. She’s very realistic as a child in that way: she doesn’t just, she just wonders, and asks questions, and shifts her worldview to accommodate new information. Adults go kicking and screaming when you try to change their minds about something, but all you have to do for most little kids is take the time to explain it the right way.
And that’s very much Diana. When an adult takes the time to explain something to her, as long as that something makes sense she’ll accept it and move on without a lot of fuss. And I think that’s what she’s done with her parents, although I’m sure a part of her still wishes that she could see both of them all the time and see them happy together.