Let's go over the German again, please :o):

You knew and never told me: Sie haben es gewusst und es mir nie gesagt!

Mystique is my mother. No, no, it is not true: Mystique ist meine Mutter. Nein, nein, das ist nicht wahr!

You are my sister: Du bist meine Schwester.

-> About "du" and "Sie": Both are "you" in English, but "Sie" is a very polite form, so it would probably fit in adresses to Xavier, but Rouge would probably be adressed in a more informal way, so "du" is more appropriate.

As for language references: I am native German :o).

Author's Response: Thanks. I used a online translation website, I think next time I\'ll ask for help instead.

Very well written, but definitely needs a sequel... please?

Author's Response: I\'m pretty sure there will be a sequel, yes. But it will be a little while yet - maybe in a month or so before the next set of prompts in the challenge. Glad you liked it!