In "Born to Run" Kate picked up a letter under the name Joan coração and cried when she read it. What did it say?

The letter was most likely about her mother's cancer. You can assume this because in the same episode she went to visit her. They did not confirm this though, so it's up to your imagination and interpretation.

I think the letter was about her mother being sick. In the same episode she went to visit her at the hospital.