According to Comic Book Movie on Sunday, Bryan Singer was able to reveal a clarification on an already assumed rumor about "X-Men: Days of Future Past". Apparently, the young girl scene in the trailer was thought to be Pietro Maximoff's sister, the Scarlet Witch, however this isn't the case. Singer just acknowledged her as his young sister and nothing more of the character's connection to the mutant in question.
So basically, she is Maximoff's little sister, but the sibling isn't the Scarlet Witch. In fact, Singer said that there was a line cut from a scene which encouraged the young sister to go "bug" the older sister. Singer said, "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".
Some even thought that the child would be returning in "X-Men: Apocalypse", but at this point it is unknown of the role of Scarlet Witch that she'll even be part of the 20th Century Fox franchise. So at this point in time, Marvel comic book fans would have to pay homage to seeing the brother sister duo team pair up as they fight in "Avengers 2". It does sound like, according to Singer, Evan Peters may be returning to "X-Men: Apocalypse".
"Even though our Quicksilver is not a main character like in The Avengers, it's a character there's a lot of thought and concern about and it's a character I'd like to explore further in the sequel".
What's interesting is the Scarlet Witch and Quicksilver characters have rights owned by both the 20th Century Fox and Marvel Studios. There's actually an infographic giving a more understandable visual interpretation of who owns what characters in the Marvel Cinematic Universe.
The difference is that in "Avengers 2" they will not even be known as any kind of mutant nor have any relation to Magneto as portrayed in the comics. The two characters were seen in the post-credit tease for "Captain America: Winter Soldier" imprisoned in chambers where they are seen in angst displaying their super human abilities.
At this point, as seen in the trailer, Logan tries to acquire Quicksilver to join them in a jailbreak to release Magneto from the Pentagon. Notably it's interesting to see the trending video game technology of that area to be that only of a stand-up arcade "Pong" game.
"X-Men: Days of Future Past" is scheduled to be released May 23.