Still up in the air, arguably. I actually think the reason why she's letting the big brother read it is because she's less concerned about his opinion as she is Ta-kun's.
I'm thinking she was actually being bluntly serious previously when she said she looked up to Tsumugu as a big brother herself. Nothing more is there to her than a sibling she didn't have if their families didn't live next to each other. While, somehow, Tasuku is different. She's actually scared of him seeing too much of her and being scared off by the things he hasn't seen yet that she's ashamed of...
A lot of these downer comments are because there isn't really reading between the lines, I feel. I could be wrong, but I trust my instincts and experience with reading these sorts of stories.