Text post about the Maximoffs!
So, that cut-scene at the end of the The Winter Soldier! That was Wanda and Pietro Maximoff, or the Scarlet Witch and Quicksilver. They're both mutants and their father is Magneto. Wanda's powers are kind of vague, they started out as hex powers and turned into reality altering powers. Pietro's power is super-speed.
They have a very long history in the comics. They were born in Eastern Europe to a woman named Magda, who ran away from Magneto because he's a scary person, and she dropped the twins off and ran away herself. The twins were raised by a Roma family, the Maximoffs, until their powers emerged, causing people to be afraid of them. (Which people depends on which origin comic you read.) They were found by Magneto, who had them join his Brotherhood of Evil Mutants.
They stayed with the Brotherhood for a while out of a feeling of owing Magneto something for rescuing them, and then Pietro started distrusting Magneto. But he stayed because Wanda wanted to stay. At some point, they found out Magneto was their real father, and sometime after that, they left to join the Avengers.
For a while, things were pretty nice for them. Wanda married a robot named Vision, and Pietro married an Inhuman (humans with powers from the moon, basically) woman named Crystal, and had a daughter called Luna. Then the tragic stuff begins.
Wanda wanted to have children, but in a series of complicated events that I don't really understand she lost them, and in her grief she ended up killing some of the Avengers. This led to the Avengers wanting to kill her because her reality-altering powers were out of control, and no one could find a way to stabilize her.
As a result, Wanda changed reality so that everyone got what they wanted most, including Magneto, who rules the world in this reality. Some of the mutants found out, though, and wanted to stop it, so they confronted Magneto and his family. Magneto found out that Pietro convinced Wanda to change the world so that she wouldn't be killed, and killed him. Wanda brought him back to life and then altered reality again, getting rid of most of the world's mutant population.
More stuff happened, Wanda disappeared and was found and redeemed herself, Pietro did some really bad stuff in his sister's absence, and now they're pretty much settle in two separate comic series that are currently ongoing.
In terms of the next Avenger film, very little of all that can be used, because Fox owns the rights to Magneto and the concept of mutants. So in the next Avengers film, Wanda and Pietro aren't going to be mutants and they aren't going to be Magneto's children because Magneto can't exist in that universe. (They did, in Days of Future Past, hint that Magneto was Quicksilver's dad, though. Which was nice.) I've heard that the film is taking elements from the Ultimates comics, which worries me. The Ultimates is a sort of alternate universe in Marvel comics that...gets a lot of character things wrong. The twins, in the Ultimates!verse, have an incesty relationship. And they're pretty much on the side of evil. Which, meh.
The nice thing about the comics twins is that they really care about each other, and they've always wanted to do good. Wanda tends to lose control, and Pietro is an impulsive ass much of the time, but at the heart of it they both care about their families and each other, and about being good people.
So, if you want to read some Maximoff-related comics, there are these:
Avengers Origins: The Scarlet Witch and Quicksilver, which is the most modern retelling of the story. It has some good elements and bad, but it's probably the easiest read since any comics before the 80's are kind of hard to deal with.
House of M: The one where Wanda changes the world so that Magneto rules it. A lot of the focus is actually on the X-Men, but the parts towards the beginning and the end give a good idea of what the twins' mindset was like when all of it started.
Son of M: Okay, the writers lose the plot on this one, but there are some good character moments. It focuses on Pietro after Wanda takes away his powers, and he pretty much can't deal with it. It's pretty dark, he's not in a good place mentally. It's interesting how the writers deal with him losing his powers and his sister. But there are some plot elements that are just, no. That said, the stuff in this book is a good jumping off point for understanding why people are so angry at him in later comics.
The Children's Crusade: The souls of Wanda's twin sons currently reside in the bodies of Billy Kaplan and Tommy Shepherd, who, in the past few years, have become part of the Young Avengers. When the two of them find out about their possible relation to Wanda, they set out to find her. The problem is, the Avengers and the X-Men want to find her too, to exact revenge for what she's done. It's a really nice book (in my opinion) because you get a lot of good family moments. Tommy and Billy get to learn about their family by teaming up with Magneto and Pietro, and about their mom. And the Young Avengers are a very fun team as well.
Uncanny Avengers: I haven't actually read this one, but Wanda is a main team member. The main reason I haven't read it is because by the time I found out about it, it was already a lot of comics in.
All-New X-Factor: Pietro joins a team led by his half-sister, Polaris. It's my favorite comic right now. There's team building, redemption arcs, trying to figure out what it means to be family (for Pietro and Polaris, but also in issue 14 for Polaris and Wanda) and just lots of nice character moments for everyone.
There's other comics in-between all these, but these are kind of the ones that form the basis for the Maximoff twins. Also, I didn't include anything from before 2000 because they might be harder to find, and also because I know very few of those comics really well.
Also sorry for the long post! The main takeaway is that the next Avengers film is using these characters, but I'm assuming it's going to be nothing like the comics because of rights and stuff.