
do you think the letter contains secrets on how to use her power?

He might have read about it. Since he didn't have silver hair, he didn't have to worry about being killed in his mind. That would explain why he was also confused and concerned when the carriage incident happened.
I realize returning to your abuser would be one of the last things anyone would want to do. However, many think they have nowhere else they can go. Her uncle could have easily fallen to that logic so he could have read something to meet him to believe starving to death would be better than going back. Or as he became older, he could have connected dots he wouldn't have as a child.
MINDBLOWN WHAT A BADASS FUCKING MOVEEE