Both Disney and Fox had the rights to use the twins if they wanted. That's how Fox wound up shoving their own Quicksilver into the X-Men movies. For whatever reason, they never introduced their own Scarlet Witch (maybe she's just too similar to existing characters like Jean Grey), but they always had that option and Disney couldn't have stopped them.
I would like to believe we'll see all that in the show but we don't even know yet if Agatha Harkness is actually in it or if that was just a fake leak (remember, that 4Chan leak got some things right, like Sharon in the Falcon/Winter Soldier show, but it said nothing about the sitcom/'50s TV premise of WandaVision). It would make sense if the "nosy neighbor" turned out to be her but let's not get ahead of ourselves, remember when we had all these ideas about what would happen to her and Vision in Avengers 4 and then he wasn't even in it or directly mentioned?