Why Cecelia didn't explain everything to her sister Briony?

I'm not sure that would have mattered. The scene by the fountain is explained in the book as Briony thinking Robbie had cast a spell over Cecelia to control her. (She was "rather fanciful" after all.) The movie limits it by making the audience have to figure it out for themselves by showing the moment Briony sees Robbie holding up his hand and Cee stopping in her tracks. It's only in the follow-up scene of the same incident from Robbie and Cee's POV, in which Robbie holds up his hand only so Cecelia won't step on the vase shard, that the audience sees the real reason for his gesture.
It would explain in Briony's mind why Cecelia didn't scream when Robbie "attacked her in the library." She would have thought if Cee tried to exonerate Robbie it was just more of the spell.
Now, of course, Cecelia wouldn't have known any of this so she might have at least tried.
But also, explaining she just had sex in the library to her preadolescent sister who doesn't understand sex probably wouldn't have gone over well. And Cecelia could have had only a sense of Robbie's innocence anyway. She wasn't out in the woods and a witness. Briony was, even if a bad witness.
I will admit the arrest and conviction of Robbie are glossed over a little too easily in both novel and movie. But the story was not a mystery to be solved, but of lives lived.

BTW, since the entire story is Briony's, the second scene of the same moment by the fountain was only Briony's reconstruction in her own mind what Robbie might have been doing. Perhaps Robbie DID have magical powers over Cee. 😉

Or if she had a better sense of decorum and went into a room for her tryst with Robbie, an impressionable 13 year old child would not have seen them, and been traumatized by the event.
So there was bad behavior all around.






Schrodinger's cat walks into a bar and doesn't.