In X-Men: Days of Future Past, Quicksilver/Pietro Maximoff is holding a small girl while watching Magneto on TV with his mother standing near. Now I wonder who she is. Quicksilver has a twin sister Scarlet Witch/Wanda Maximoff in the comics but the girl appearing in the film is lesser in age. Is she Polaris (which doesn't fit well too) or some other character from X-Men comics.
According to this interview with Bryan Singer:
“Is that the Scarlet Witch? No, that’s his little sister,” Singer said. “I even had a line which I cut, where Quicksilver’s mother says to the little girl, ‘Go up and bug your sister,’ and the little girl says, ‘She bugs me!’ You never see the older sister, but it was to imply that there is an older sister for comic book fans. I ended up cutting it.”
The little princess from Days Of Future Past is Lorna Dane aka Polaris. She's the only sibling we know from the twins and who is also Magneto's daughter.
Bryan Singer stated that little girls is not Wanda because she's the twin with Quicksilver. She's not just a little girl either, because that was Polaris and yes, Lorna Dane is younger than the twins. Bryan Singer did state he did it for the fans. As I recall perfectly, the fans are into Polaris, Quicksilver and Scarlet Witch. Singer doesn't care about the mythology of comics.
She was referred as a Princess before so that can be a hint that it was Princess Lorna Dane.