This week's episode of WandaVision, "Previously On", answered a lot of questions about what is going on with Wanda and Westview. After the reveal that Agnes is actually Agatha Harkness (Kathryn Hahn) in "Breaking the Fourth Wall", this episode saw Agatha question who and what Wanda (Elizabeth Olsen) is while revealing, by walking her through her past traumas, the nature of her magic as well as how exactly Wanda ended up warping Westview into her own personal sitcom. However, while the episode connected a lot of dots, there are still some major questions that remain and one of them is huge: are Wanda's twins real? It isn't something with an easy answer, but it seems likely that Billy and Tommy could in fact be real beyond simply being "real" to Wanda -- but how sustainable they are remains to be seen.
One of the biggest "clues" that the twins are real comes from the nature of Wanda's powers themselves. One of the things that is made clear in "Previously On" is that Wanda is, through her magic, capable of spontaneous creation. We already sort of knew that -- the whole Westview situation exists, after all -- but Agatha's explanation of what is going on puts a bit of a finer point on it as Agatha is questioning how Wanda can even do it to the extent that she is. The very idea that Wanda is "creating" suggests that her creations are very real. That's somewhat bolstered by the death of Sparky. If Sparky wasn't real in some sense, Agatha wouldn't be able to have killed him.
Another indication that the twins are real are Monica's (Teyonah Parris) clothes as "Geraldine". When Monica was ejected from the Hex, not only did the clothes that she was wearing not disintegrate, they also didn't change back to what Monica was wearing when she first entered the situation. While one could argue that the clothes were simply a manipulation of something that already existed, even then there are elements of the clothing that likely didn't exist before the Hex. If the clothing is real in the sense that the changes Wanda made to them persist outside of the Hex, it would make sense that anything else created could also continue to exist outside the Hex as well.
That brings us to the obvious exception to that, The Vision. As viewers saw, Vision began to come apart when he exited the Hex which would seem like a strong argument that Billy and Tommy also would not be able to exist or be "real" outside Wanda's world. However, Vision is unique in that we now know that Wanda didn't simply put his pieces back together and bring him back to "life". The Vision who is in Wanda's Westview is something that she completely created and the real Vision does still exist in the real world -- and is being recreated by S.W.O.R.D. One could argue that the reason Wanda's created Vision comes apart outside the Hex is that he's a duplicate that cannot exist while the real Vision also exists. Had Wanda's Vision been made from the real Vision or the real Vision didn't exist in the real world, there may well have been no issue when he breached the perimeter.
If the twins are real, the question then becomes how sustainable they would be outside the Hex and that's where things get tricky based on the details of their creation. In comics, the twins were created from a portion of Mephisto and when that was reclaimed, the boys ceased to exist. If Wanda did use some sort of energy or power outside of her own to create her children, it's possible that as things grow more complicated with the idea of different realities and powers, whatever is the source of Billy and Tommy could disappear or be reclaimed, ending the existence of the boys and thus causing Wanda even more pain. It happened in comics and that's something that fans of the comics know all too well where that kind of pain for Wanda leads.
The first eight episodes of WandaVision are now streaming on Disney+