After he had asked them to go to their rooms, they disobeyed him, it was his fault to do so, and as he lured his sister to the hidden part of the house that unless you wanted to swim, no one goes there, absolutely no one.
The secret he is busily accusing his own about is the one about a man and a woman in a room making babies.
This is absolutely absurd, and the more he tries punishing his own, the more she also tried reasoning the matter with him.
"Hey, you know I couldn't lie to him, he would have punished me, and you know th way I'm weak when it has to do with punishments. Lucky you, he didn't flog nor punish you," he said trying to bring correct reasoning to her his head.
What she said was the truth, the absolute truth, and there is nothing bad about it, just that she has an elder brother who thinks he knows everything.