Maybe I didn't make myself clear.
Take a look at the last couple letters:
March 13, 1998
This is the last time I am writing to you. I am not sorry for what happened. In fact, I am glad that it happened as it made me realize that I care about you more than what I should feel for my brother's best friend.
Take a look at what I've enclosed, and tell me what you think.
March 20, 1998
Think about it. Fred and Hermione are exchanging letters, obviously growing close if Hermione knows Fred well enough to tell him something she hasn't told either Harry or Ron. When she visits him in Diagon Alley, something happens--whether it's sexual or just a simple kiss, you don't know. What you do know is that it's making her question their relationship, and she's not comfortable with what she's finding.
She won't answer Fred's letters. Ron finds one of them and talks to Fred. Ron gets Fred to send Hermione one more letter, and Ron convinces Hermione to read it.
Fred makes one last plea for her understanding, signing his letter, "Love, Fred." If you look back, he started signing his letters that way after she visited him in Diagon Alley. Hermione never signed hers that way.
Fred says in his letter: "Take a look at what I've enclosed," clearly telling us that he's enclosed something in the letter beyond the letter itself, which is all we see. He also tells her to tell him what she thinks.
She says "Yes," and signs her letter, "Love, Hermione," indicating that either her feelings have changed, or she has accepted her feelings as they are. I suppose that simply saying "Yes," confused a lot of you, as became apparent after I began receiving reviews.
The key is in the sentence, "Take a look at what I've enclosed." What else could he enclose that she would say "Yes," to?