Briony is such a very naughty girl. She ought to be grounded. Not only has she read the letter that Robbie never meant for anyone to read, she's now gossiped it to her flighty, melodramatic cousin. Oh this will end poorly.
Awesomely, Briony (having no understanding of physical attraction whatsoever) has decided that all of Robbie's actions this day are dangerous threats of violence. I find this hilarious. She thinks her sister needs protection from this terrible maniac.
Her fears are confirmed when she discovers Robbie "attacking" Cecilia in the darkened library, shoved into a corner, hands pinned over her head, etc, etc. Sounds terrifying. If we could all be so unlucky. Sigh. This seems to indicate that the bluntness of Robbie's letter has erased all the awkwardness between him and Cecilia. That's a relief. Now all that is left is for this all to somehow get back to the parentals.