Debra's letter was read aloud to the family in the episode "The Letter".
Here is the part of the letter that Frank read aloud:
"Dear Marie. This letter has been a long time coming. For eight years now, I've held my tongue and never told you how hurtful and destructive your behavior can sometimes be. Just because we are family, and happen also to be neighbors, does not give you the right to constantly interfere in every aspect of my life, from raising my children to my choice of liquid fabric softener. I'm sure you don't even realize when you're being overbearing, critical, and intrusive. ... "