But they clearly want to keep as much of their own rights out of the hands of other studios so they can keep the creative control over them. If they and they alone were having the talks of wanting someone to play Wanda, then they would have the sole rights to her.
And where was anything saying she was confirmed? If anyone wasn't named in the films or didn't have a part of any significance, then they were open to interpretation of who exactly they were. There may have been allusions to them, but if they weren't named, then Fox likely doesn't own the rights.
And why do these two even need to be in these films? I keep seeing these two come up every so often, but I see no significant argument other than "But they're Magneto's kids." They weren't a part of Magneto's life until they were nearly into adulthood, so they had no significant part in his origins. They had no bearing in how he ended up the way that he did, nor did they shape his decisions leading up to it. Putting them in these films would be doing it just for the sake of doing it, and would stop Marvel from being able to use them where they actually had an impact in Marvel history with the Avengers until Marvel gets the rights back from Fox.