When the picture was first released of X-Men: Days of Future Past Quicksilver sitting with a little girl in a pink princess dress, the internet blew up.  Mostly because those of us who know our X-Men lore know that Quicksilver and The Scarlet Witch (Pietro and Wanda Maximoff) the children of Magneto are TWINS.  Twins.  That means the same age.  There was a lot of debate about whether or not they would have Pietro age quicker in the film, and that was the explanation for the age difference.  But now, Bryan Singer dispels this in an interview with Empire Magazine:

“Is that the Scarlet Witch? No, that’s his little sister. 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.”