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: