I'm going to try and stick to facts-as weak as they are.
Being alive, though not determining, is a plus. It's one bit harder to coherently revive a character than not to. Johnny and Sonya will have to be modeled to some extent to be present in the story. Yes, their daughter is a mixture of them both.
Don't you think they could still make both of them with little changes in their gameplay? When on earth has Sonya called for missiles? Or shot a gun? The only similar move is the take-down, which she only got in the last game, and a (backwards) flip.
Boon said about fatalitites "someone said; what are we going to do about fatalitites on family members? And I said, oh, I haven't thought about that, we're probably going to have them anyway (the fatalitites on family members)".
We've had Sindel and Kitana. Two Sub-Zero brothers and so on. So even if they're in, he's not talking about them. He's talking about earthrealm heroes, humans killing their daughter. (mother/father/wife). Keep in mind this was said while presenting Cassie. Why would you pay attention to his words, you may ask. Because "Sub-Zero A might be a good matchup for Raiden B".
They're alive. They aren't similar enough to Cassie, especially Sonya. They are already in the story. They could be as young as 26 and 30 or as old as 51 and 55 or anything in between. They are both extremely popular. (I believe, at least quite popular, from the original seven).
That's why I think at least Sonya will be playable.