The following is a longer explanation of a prompt posted to the Secret Mutant Exchange on A03:
Sooo... I've been wondering what would happen if/when Charles discovers that Kurt is his nephew. He doesn't have any kids (that he knows of) and he isn't exactly getting any younger. Furthermore, he's almost died multiple times and let's face it -- his chances of having a child now in his current condition are slim to none. So when Charles discovers that his sister's child has joined his school and wishes to be part of the X-Men, does he legally recognize Kurt as family? Offer to place him in the will? (And if so, does he get Hank do a DNA test on the down-low at first just to be sure?)
Or does Charles keep his knowledge a secret and make an effort to treat Kurt no differently than anyone else, since Mystique seems unwilling to recognize the child herself? And does this eat him up inside, or is he secretly relieved because of the way that Kurt looks and how society would react to that being the next Xavier heir?
I'm happy with whatever way you choose to go with this, dark!Charles or heartwarming!Charles.
I definitely think Mystique has major issues, the only question is whether Charles would encourage her to stay distant and detached or encourage her to forge a connection with the boy. And that depends entirely on how jaded you feel about Charles, honestly. I want to think that Charles has grown and changed enough to publicly claim Kurt as immediate family. But would that be politically wise given how close they were to Sentinels and extermination camps?