In the video game X-Men Legends II: Rise of Apocalypse, Magneto spends the majority of the game worrying about the safety of his son Quicksilver, but does not appear to be at all concerned about the well-being of his daughter Polaris. My question is why? In the comic books both Quicksilver and Polaris are established as Magneto's children, as well as the Scarlet Witch. Any answers would be greatly appreciated.
TL;DR : While it is true that Magneto's relationship with his children has always been very complicated, he has always seen Pietro as the one who would carry on his legacy.
Even before he revealed the relationship to the Twins, Magneto was adamant that the two - especially Quicksilver - believe in his cause of mutant superiority and work to further that cause. Magneto was devastated when the pair left his Brotherhood of Evil Mutants, going so far as to kidnap them and make them return by force (Avengers Vol.1 #48 way back in 1968):
In fact, it was largely because of his children that Magneto turned over a new leaf and attempted to achieve his goals in a peaceful manner. It was his hope that if he could strengthen the relationship between himself and Pietro, thus convincing the boy to follow in his footsteps. In fact, no matter how rocky their relationship at the time, Magneto has used every opportunity as a chance to mold Pietro in his own image:
Magneto has never been particularly close to Polaris, which is ironic given that she is the one child out of three who accepts their relationship.