There is some talk about Billy being a pure soul, but that doesn't quite track with his behavior, and I think a smarter move would have been to make him a choice of desperation due to Strong's attack.
It was clear to me that this was the case. I asked my daughter about it and she agreed.
I'm not terribly familiar but when I learned the older sister's name I knew immediately who she was and figured that it would either happen at the climax of the movie (her only) or they would tease it for a second movie. In fact, at the climax I expected her to interject herself in the confrontation, grab the staff, etc.
In the original comics, Captain Marvel and Billy Batson are distinct individuals and do not even share memories of what the other does, which occasionally caused problems when Billy would change and CM would not immediately know what was going on or why he was summoned. I believe the comics are also using this 'CM is Billy in another form' version now, which made for a better movie.
