Warning! Spoilers for Marauders #1 ahead!
The origins of Cassandra Nova, Charles Xavier's "evil twin", have always been a bit of a mystery in X-Men's lore. Created by the visionary genius of Grant Morrison for their New X-Men run, Cassandra is technically a "Mummudrai," a parasitic spirit from Shi'Ar mythology. She formed in Charles' mother's womb during her pregnancy and stole his DNA, using it to build a body for herself. The question for years has been if this act made Cassandra technically a mutant or not, and it has been finally answered thanks to some of the finest scientific minds on Krakoa, as shown in Marauders #1 by Steve Orlando, Eleonora Carlini, and Matt Milla.
Mummudrai, also known as the anti-self, are entities that each individual must fight before being born, according to Shi'Ar myth. This spirit is the equal and opposite of a person, but usually it does not take corporeal form. However, in Charles Xavier's case his amazing genetic potential allowed him Mummudrai to assume a form, effectively creating a twin. According to Xavier, Cassandra tried to strangle him in the womb. Sensing her evil intent, Charles preemptively tried to kill her with his nascent psychic abilities, forcing his mother to have a miscarriage. Although pronounced stillborn, Cassandra in fact survived and spent decades as a growing mass of cells in a sewer wall, building a new body for herself and planning revenge on her brother and the entirety of the mutant race.
Cassandra is responsible for the greatest crime in mutantkind's history, the genocide of sixteen million mutants on the island of Genosha. She had many confrontations with the X-Men, and her incredible psychic powers and the ability to switch bodies made her almost impossible to contain. At some point, she took over Charles' body and used his relationship with the Shi'Ar Empress Lilandra to almost destroyed the Empire from within. Because she built a body based on Charles' DNA, Cassandra is technically a mutant, even if her origin as a Mummudrai is completely different. This debate has polarized fans for many years, but it was nothing more than an academic discussion until the foundation of Krakoa made Cassandra's status an actually relevant subject.
The basis of the nation of Krakoa is that every mutant in the world is given an amnesty for his previous actions. This is is the foundation the mutant united front that sustains Krakoa, with all its contradictions. Criminals and mass murderers such as Mister Sinister or Apocalypse were given a clean slate, and even a seat on the Quiet Council, the ruling body of Krakoa. However, Cassandra Nova is the most infamous criminal in mutant history, so her access to the amnesty was bound to create controversy. For that reason, a scientific panel consisting of Mister Sinister, Doctor Nemesis, and Doctor Cecilia Reyes is assembled to pronounce a final assessment on Cassandra's belonging to the mutant species.
While Sinister is in favor of considering Cassandra a mutant (because he also was not born such, but became one through scientific means), Nemesis is opposed. In the end, Doctor Reyes tips the scales, saying that Cassandra's act of copying Charles' DNA was not conscious so, one way or another, she was born a mutant. This official stance solves a decades-old debate on Cassandra Nova's status, but at the same time poses a great moral dilemma for everyone on Krakoa. It will be interesting to see how the X-Men and other mutants will react now that the genocidal Cassandra Nova has been officially pardoned and even joined Kate Pryde's team of Marauders.