She is Magneto's daughter, yeah. Magneto didn't adopt her, but her mom did have an extra-marital affair with him while married to the man Lorna assumed was her biological father. They died in a plane crash as a result of something Magneto and Mastermind did in regards to Lorna and suppressing her powers/memories.
Great artwork! So Polaris turns out to be Magneto's daughter after all these years? That means he has three children now? Her and The late Scarlet Witch(she'll be back, they ALLWAYS come back!)and Quicksilver!