WandaVision's third episode introduced a pair of two new characters: Wanda and Vision's twin sons, Tommy and Billy, who are born after one very odd (and fast) pregnancy. Comics readers will know that these aren't inventions of the TV show. In fact, they're at the centre of one of Wanda's biggest storylines, and they may hold the key to unraveling what, exactly, is happening in WandaVision.
In the comics, Wanda and Vision's relationship survives long enough to actually get married, just as they're depicted in whatever reality WandaVision is taking place in. In one storyline, they take a leave of absence from the Avengers, and, thanks to the assistance of a magic spell, are able to have twin sons, who they name Thomas and William, just like in WandaVision. Their happiness doesn't last for long, however: Vision goes through a horrible series of events that ends with him being dismantled and then reassembled, and when he returns, it appears his entire personality is gone. Eventually, Wanda learns the truth: their sons are nothing more than shards of the demon Mephisto's soul, and once he "reabsorbs" those shards, Wanda and Vision's children are lost.
Following this tragedy, Wanda slowly descends into madness. At one point, her memories of her children are erased by her mentor, Agatha Harkness, but the memory block doesn't hold up. Instead, she seeks out the help of supervillain Doctor Doom to try to bring her children back to life, and he helps her merge with a cosmic entity as part of her attempt. While under its influence, though, she tries to destroy the Avengers, blaming them for her sons' deaths, and the resulting conflict kills both Vision and Hawkeye before she's finally stopped by Doctor Strange. In an attempt to save her from being harmed by the Avengers, her brother Pietro convinces her to warp reality to create a universe where everyone gets their hearts' desires. The reality doesn't hold up, and eventually, Wanda goes into a secluded life without full, true memories of her past.
That's not the end of the story for Wanda and Vision's children, though. Two members of the Young Avengers — Wiccan, who possesses powerful magic, and Speed, who has speed superpowers like Wanda's brother Pietro — discover that they're reincarnations of Thomas and William. It remains to be seen what will happen with these versions of the twins, but if the on-screen universe is anything like the comics, there's quite a bit of story left to tell.