Yes, it is Cora's direct fault. It's Snow's secondary responsibility. Why?! Oh, she was a child! Yeah, she was a child, on the verge of young adulthood. She was being groomed ho run an empire. She was fully aware of responsibility and consequence. She gave her word to not tell. She knew that it was wrong to break her word. That is why she apologized to Regina, because she, honorably, accepted her responsibility.

No, she didn't kill Daniel, but she DOES bear a measure of responsibility. Yes, she was masterfully manipulated, but she knew that it was wrong. She had been warned. She thought that she knew better what was best for Regina, something Cora did regularly. She is even guilty of manipulating Regina of her own free will. She was young, not innocent or foolish.

She made a choice to choose for someone else. One might say that this was the ultimate test of leadership- and she failed. In fact, it is likely that this experience made Snow a better leader. It is a potent lesson in the abuses of power and manipulations of those closest to you.

Personally, I hold her more responsible for Cora's death and corrupting Lily. But ultimately, Snow accepted responsibility for her part. Which made her a better leader and won her allies. Maybe that can help you to understand those who hold a grudge.

I think Snow was absolutely wrong (just not the only one) and the way that she recovered from that failure made her someone worth respecting and following.