It should have been so simple, just make some friends, find the elements and beat the villain that tries to make the night last forever, then find out she's Celestia's actual sister and absolve her as they hug… Well, perhaps not simple per say, but expected.

So then why when they defeated Nightmare Moon did Luna ignore her sister Celestia? And why did she instead hugged Twilight as if she was happier to see her then her sister? Why was it Celestia who soon after groveled and begged Luna for forgiveness, rather then the other way around? And what's with this godmother thing?

For that matter, what else changed?