"Eure Schwester wird das Geheimnis erfahren."
Übersetzung:Your sister is going to discover the secret.
Kann man auch sagen: Your Sister is going to find out the secret. ?
Your sister is going to find out the secret. - müsste doch auch gehen.
know != learn (about) != discover
Even though I am pretty content with the German-English course (and vice versa), there is some serious lack of vocabulary knowledge. Otherwise: great job, but it may be the German in me putting things as bluntly. No offense meant, but the translations offered are not sufficient. Here is the basic on-the-fly distinction:
to know: (1) wissen, kennen -- to (already) have knowledge about
to learn (about): (1) erfahren -- to gain knowledge (by being told or experiencing through a subset of one's senses) (2) entdecken, feststellen, herausfinden -- to gain knowledge by some cognitive activity
to discover: (1) entdecken -- to gain knowledge by some physical activity (2) entdecken, feststellen, herausfinden -- to gain knowledge by some cognitive activity