Illegally downloaded music in Germany?

okay so I’m currently staying with my brother in law, and a few months ago I downloaded some music from frostwire. He recently got a letter in the mail and we think he is being sued for what I did. I’m freaking the out. The letter looks pretty legit like it really came from a lawyer but it wasn’t hand delivered and signed for like most legal documents. I’m really freaking out, I don’t want to go to jail and I don’t want him to take the blame for something I did. Please help! btw We’re living in Germany. Guess that’s pretty obvious from the title of the question.