Yes, Ashe hid some things from Rachel, but only those that were too personal for her could make her feel shame or ruin her relationship with her sister. But here there is no such thing. And yes, Ashe literally said that it is normal for sisters to share secrets with each other. This is precisely the logical conclusion with which she justified Fiona’s action for herself. She didn't find it unacceptable or a betrayal.
Why does the burden of proof suddenly fall on Delilah? There's nothing there to indicate she was lying. It is you who needs to prove that she was lying, given that she initially did not know that Fiona knew the secret, which means that by telling Ashe that she knows everything, she could expose herself as a blackmailer, which is a rather stupid act for someone who was able to organize blackmail. And she doesn’t look like a red herring either, since she should at least look like a blackmailer, but nothing indicates this at all. Your speculation about lies is based only on subjective feelings. Don't impose your subjective feelings on me, give me logical arguments.
I specifically left out the talk about blackmail since I know your opinion on the subject and it is not the point I want to make here. Even if she is not the blackmailer, her behavior is a bit sus when she tels Ashe that she knows the secret through Fiona. And here the dilemma is between the trust in Fiona and Delilah frankness. If Delilah is telling the truth about the source of her knowledge it makes Fiona an unreliable friend. If Del is lying then Fiona knows which secret to keep. Also Delilah could be lying simply because she is embarrassed to admit that she spied on her and Zoey. It does not have to go farther than that. the lie itself is not a prove that she is the blackmailer. That is not the point of this specific argument. It is simply a trail that must be followed.
If during a private discutions with my cousin he comes out to me as Bi I wont go to my brothers to tel them right after he told me. I would feel privileged to be someone he shared his secret with. I would keep his secret until I knew he made it public since it is his decision to make, not mine. I would want to be worthy of his trust. I would feel that telling my brothers would be a breach of his trust.
The burden of proof falling on Del comes from her body language when she told Ashe that it was Fiona that told her right after Ashe mentioned Fiona among others that know her secret. I also trust Fiona to keep Ashe's secret even from her sister. And that burden is for the players who don't believe her not for Ashe since Ashe believed her in the end. The hard evidence you want, I don't have it. For me it is not necessary to come to the conclusion that there are strong chances that she is lying. For you, like you pointed out, you need harder evidences.
At the end I talked about red herring, a trail that you do not see Delilah leading us on. That is ok, but if others think that she is a red herring or the culprit then she is at least a red herring since she is leading readers, not all readers, to believe that she is the blackmailer. Not all readers will take the bait, maybe you didn't, too soon to tell. The fact that others, not you, make her their prime suspect regardless if she is guilty of blackmail or not by definition makes her a red herring. Arguing that she is not is like arguing about other people's preference, it is a fact you can't change simply with your opinion. IF you want you could call everyone's suspect a red herring.
Look here I am trying to find common ground, I am not trying to tell you that you are wrong since the hard evidence you speak of does not exist yet. I am trying to find facts we can agree on not ones we disagree on.
I would rather do a silly tier list than argue on the internet, but I think that finding that common ground is important to build something positive on it rather than be at odds on everything.
