She used to be convinced she was stupid just because she struggled so much with reading.

I didn't find out I was dyslexic until I was in graduate school. Learning that was officially true changed my life. No, seriously. Suddenly I wasn't just a dumb guy who couldn't always tell left from right, or who was lazy because he got tired when he tried to study for long periods of time.

It would be worth it for your sister, to this day, to know this. Well, in my opinion.