https://www.ign.com/articles/wandavision-theory-the-mind-stone-is-the-key-to-everythingSummary: The fact that Vision is actually alive and has a Mind Stone in his head explains a lot. Wanda had to have pulled the Mind Stone from another universe where it wasn't destroyed (and she probably had help). That new Mind Stone can't exist in our universe, as described here:
"The comic book version of the Infinity Stones only work properly in their home universe. If the MCU follows this rule, then Vision can only be alive in the world where the Mind Stone was taken from. That could be why Wanda created the Hex. It's a bubble containing a breach into the other universe--a safe space for the Wanda-Vision family to live an idyllic life."
So Wanda is harnessing the power of the Mind Stone to control people and create the Hex, and she's going to somehow merge those two universes, likely giving birth to mutants in the process. Shark commercial indicates Mephisto is leeching on her power as well.
Pietro/Peter is likely from that second universe. Not sure if him being Mephisto makes sense because he's showed off his super speed. Not sure how Mephisto could've pulled that off.