Counter idea: Wanda and Pietro are Magneto's kids, but chaos magic prevented them (or at least Wanda) from inheriting any mutant genes, meaning the twins (or at least Wanda) aren't mutants but still count as part of the House of M.
It would actually be quite interesting if either chaos magic impacted Pietro this way due to being Wanda's twin, or alternately if they have to grapple with one being a mutant and the other not despite being twins.