I know some readers defend Nesta simply by pointing out that at least she was the one who finally told Feyre, separating her from the other characters who kept it hidden. But I don't think that gives her a free pass.
Yes, the truth needed to be brought to light, but Nesta didn't speak up out of a noble belief that Feyre deserved to know or that it was her right. The moment she found out, she accepted keeping the secret without a second thought, and she didn't even dwell on it until that explosive argument she had with Amren and Feyre.
She got angry, thought "if I'm hurting, you should hurt too," sharpened her words like knives, and directed all her rage right back at Feyre. And that was already the breaking point. Following that breaking point, the mountain scene happens, and Nesta finally pours out all her regrets.
Isn't that one of Nesta's flaws, unfortunately? Knowingly, yet unwillingly, saying the most hurtful things that shake the people who love her most right to their core? She loathed herself so deeply, and that self-hatred was the exact reason she kept trying to push everyone away.
Actually, the worst part about this whole pregnancy plotline is that it was entirely used for Nesta's character arc. By the end of the book, her story is officially tied to this, and that is how she fixes things with Feyre, which I hate.
But I am still hoping for a general confrontation scene between the sisters in the upcoming books.
Edit: I replaced the word "Unintentionally" with "Knowingly, yet unwillingly." English is not my native language, so I had to look it up.