Question***SPOILER* **

Yes the sister was lying, and thats why they destroyed the letter. So no one would find out about it and it would remain a secret between Eve and Cisley.

"Memory is a selection of images, some lucid, others printed indelibly on the brain. Each image is like a thread, each thread woven together to make a tapestry of intricate texture and the tapestry tells a story and the story is our past"

This is the final quote at the end of the movie when our brave little character Eve finds out the truth of what really happened. I believe what Kasi Lemmon's was trying to tell us through images and words is that two memories are never the same. The character Louis (Samuel Jackson) clearly states in the letter written to Mozelle (Debbi Morgan) that he is offended that she accused him of abusing his "most beloved child". He goes on to say in the letter that the first kiss was "a sweet indulgence" and in his "scotch haze" it took him a moment to realize that his daughter was kissing him passionately like a woman would. Now, later on when Eve confronts Cicely and takes her hands in hers, her sister finally states that she can't remember what happened. Now, obviously Mozelle accused Louis of sexually abusing his daughter and I believe she makes this accusation because she saw a vision. Now let's also go back to the beginning of the movie for a important scene that the director made you very aware of. The scene happens at 41 minutes into the movie. The father is at the dinner table with the family and they're talking about Mozelle and he says that she "isn't unfamiliar with the inside of a mental hospital". He goes on to say that "Aunt Mozelle is a little eccentric and fortune telling is just something we let her do to keep her out of trouble." The point being made from this scene is that Louis doesn't believe in Mozelle's vision's. That being said, Louis doesn't believe that Mozelle is telling the truth-she is "accusing" him of sexual abuse. Let's also remember that Louis probably wouldn't remember what really happened because he was in a drunken haze. Now let me also make the point that Mozelle and Eve's visions aren't memories, they are things that will happen or have happened, they are the truth like when Eve sees the car accident that kills her uncle Harry. That's also why each time when Eve won't tell her Aunt Mozelle the truth she tells her to give her her hands. The final point I'm trying to get across is that Cicely doesn't remember all of what happened (probably because of trauma)and wakes up the next morning and has blood on her underpants. The father doesn't remember because he was drunk. That being said both of these are hazy memories, when Eve touches her sister's hands she sees that her sister said (what sound's like) no and her father raped her anyway. When she takes her hands away she realizes that her sister was telling the truth and that the actual account was far worse than what her sister ever told her, but she doesn't remember because she probably blocked it out. The sister also says "I can't remember" and she follows that up with "I can't". The final quote that I'd like to follow this all up with is when grown up Eve, who also narrates the above quote says this after she finds out that her sister doesn't remember the rape.
"I have the gift of sight but the truth changes color depending on the night and tomorrow will be clearer than yesterday."
I believe this quote means that Cicely cannot see the whole memory today but will eventually remember it. I'd also like to say that I felt through the whole movie that the father, Louis was a little too close to his daughter the whole time and I felt sexual abuse coming on even before it happened-Was it premeditated? I think so, especially when he would dote on her, dance with her, send her to the beauty parlor to get her hair done and have her make his drink for him when he got home...but that's another board topic onto itself.