The best explanation for how to make Wanda and Pietro mutants and Magneto’s kids again is the same one Marvel has been using for ten years–Wanda did it.

In the final moments of House of M, Wanda rejects being a mutant, rejects being Magneto’s daughter, and then performs a Omniversal-level reality warp to get her point across.

Next thing we know, Wanda is the daughter of a woman who looks identical to her, who’s really a Maximoff, who is the mother to Pietro as well, who’s entirely a witch and not a mutant.

Who bears the Scarlet Witch name, transforming the Scarlet Witch identity from a mutant

moniker given to her by hateful and fearful peasants to a rich legacy for Wanda to live up to.

To give hope at her lowest point, Wanda made the most ideal version of herself.

Now she needs to accept this part of herself again that she’s been implicitly rejecting since House of M, and basically any cosmic retcon magic in the book can get things back to normal.