They might have put the matter before her so that she would keep an eye on Sidonie and talk seriously to her.
"Indeed, that's a good idea," Sigismond interrupted.

"You must go to the Rue du Mail and tell her parents.

I thought at first of writing to little Frantz.

He always had a great deal of influence over his brother, and he's the only person on earth who could say certain things to him.
But Frantz is so far away.

And then it would be such a terrible thing to do.