I put "My mother is pretty." What was that marked wrong? According to Word Reference and Larousse - belle can mean pretty or beautiful.
I think that jolie/joli is usually used for pretty, from what I understand, and belle/beau are more commonly used to mean beautiful. I could be wrong.
There is nothing to fix. On Duolingo, "belle" = beautiful, "jolie" = pretty. It is a convention used on Duolingo to emphasize the two different words.
My French teacher taught us a trick for accents on the family. She said that they're all accent grave, going down, because one day your family is all going to go down to their graves. Which is a pleasant way to remember your accents.
I thought joli(e) meant lovely/pretty while beau/belle meant beautiful.