Often when talking about "Why did everyone treat Elain differently than Nesta," I'll see that they were different: Elain was respectful, Nesta hostile, and thus you'll get treated the way you treat people.
The thing is, the same can be said about Feyre and why Nesta treated her differently than Elain.
Let's take their first fight on the page, when Nesta says she expects to be proposed to. Feyre thinks that Tomas isn't a good candidate because he's as abusive as his father. Aloud, she says that Nesta is useless and they don't have money for her dowry.
In MAF, she comes to visit them for the first time, she brings unknown males and wears a crown (for zero reasons at that moment), and she gets "oh poor me." at Nesta's normal reaction.
In FAS/SF, she constantly invites Nesta to her new family soirees and makes zero effort to include Nesta in the discussion, ignoring her and leaving her without a gift. She also leaves her alone without a portrait on the wall of fame. After she accused Nesta of tampering with their reputation, she starts again her song, "my sister hates me."
Obviously, Nesta isn't blameless in this situation; her response in TAR was too harsh and made to hurt, and in general, she's often cold with Feyre. The idea is - people need to stop treating Feyre and others like martyrs for having to deal with Nesta. They aren't better.
For example, Azriel, unlike Cassian, didn't jump on the "let's hate Nesta because she's complicated" train, and, oh, surprise, Nesta treats him respectfully in response. Unlike Feyre, he thinks that maybe both sisters deserve gifts. And he doesn't fear to offer her gifts in public, and he receives a hug because Nesta isn't a monster, and I'm sure if he prepared something in FAS, she would accept it despite her depressed state.
Hate Nesta all you like, but the problem in their past relationship wasn't just Nesta, and it wasn't just Feyre, and not even just their father. It was an unfortunate case of a dysfunctional family, and sometimes the only thing that can help is an open discussion, some time, and space. And treating Nesta bad and Elain nice because of their relationship with Feyre is still questionable behaviour.
