Rachel summer's the bright lady
Is rachel the most important figure in mutant and x-men history
Without rachel being the mother Askani you don't get the birth of cable, En Sabah Nur becoming Apocalypse, Nathaniel Essex becoming Mr. Sinister who have guided and been involved with the mutant race in the modern era.
Sinister has been involved with many of the x-men even charles himself.
So if charles is the father of the x-men rachel is there mother