Ok, Sarah is totally losing it, but I think the main reason she's acting like this is Jim. I'm sure he must have been talking non stop about Tanya for the last couple of years like a freaking psycho (how many comments on her IG, how many Q&A refering to her) to the point Sarah started feeling jelous and insecure about herself. Add inmaturity and hormones and you have this situation. I'm sure he pushed her to write that story as much as I'm sure he's now telling her he never told her to do so as he wanted to keep it private, so she's feeling ever worse.
As much as I think she didn't have the right to split the tea, I feel sorry for her and her mental health. And for Tanya, if she cheated on Jim, well, can't blame her honestly.
( English is not my first language )