This definitely wasn't going to be this long (an intended paragraph), but here we are. I was just curious as to why some Archeron’s get along better than others, and then I digressed into Nesta and Feyre and Dark!Elain....
I believe that Nesta and Elain work so well is because they’re incredibly similar. Where Nesta builds her walls a mile high and three feet thick to keep others from seeing her insides, Elain does the same thing except she decorates hers with flowers. Not a lot is known about either character, and sure, that’s a consequence of not having their perspectives, but Nesta is aggressive to hide her feelings and Elain is nice to do the same. In the case of Nesta and Elain, like calls to like and their relationship may stem from them hiding their feelings.
This is opposed to Feyre who is more physical with her emotions. After she crosses the wall, she fights through the trials, throws the spear at Amarantha, explodes with her panic attack, wears through Cassian’s punching gloves, shoves Rhys to the ground after he used her as bait with the attor, chews Rhys out for hiding the mate bond, paints the cabin, kills the attor with burning malice, etc.
I’m not saying that Feyre’s actions are better or that Nesta and Elain’s are, but what I do see is that the two sisters who stuff and hide their feelings are the ones who bond more. Nesta’s protection of Elain may have less to do with how fragile Elain is, and more to do with how she sees herself in Elain. She understands Elain’s need to hide herself, and I think inside Nesta deeply wants to understand Feyre. That moment when Nesta says, “That was why you painted the night stars on your drawer,” is the moment when she finally sees her sister.
The significance of Feyre painting that image way back when is another physical representation of her feelings, and I think Nesta realizing that could be the moment when Nesta gets it. Her sister is not one to build walls, she acts out everything she feels inside. To supplement their possible relationship development, when Feyre is on the bridge with Cassian she describes Nesta as feeling too much saying that “She comes across as rigid and vicious, but I think it’s a wall.” (my italics)
There is no coincidence that Feyre’s story begins once she crosses a physical wall, Nesta hides herself behind an emotional wall, and now all the Archeron’s have crossed said physical wall.
When Nesta looks for Feyre, she sympathizes with Feyre a little at the wall. And I believe that Nesta will be doing more (emotion wise and physical plot involvement wise) in the next book which will cause them to grow closer.
But in addition to that, I said before that Elain built the same walls like Nesta except she decorated hers with flowers. Which first of all, her constant emotional state is almost always polite, which she chooses in contrast to Nesta’s anger. Second, her fiancé literally has walls built around his property. Now while that obviously is symbolic of her lover, it is symbolic of Elain as well. She wouldn’t be attracted to that unless Maas wanted to demonstrate this whole internal/physical wall thing and how Elain needs to move through two walls. Going off that, Nesta admits to Feyre that Graysen is fine, except his father and the wall which Nesta “thinks… looks like a prison”. Nesta’s statement along with the idea that she approaches The Wall marks her, perhaps subconscious, attempt at change. (*cough cough* letting those emotional walls down *cough cough*)
Which leads me to my last idea: Elain could very easily take longer to admit how she really feels inside. People like to assume it’ll be Nesta, but I think Elain’s got some baggage. Maybe it’s because when something inside is darker, people hide these things harder. But Elain has shown no indication of trying to expand beyond what she hides in her mind finding solace in a man whose property is a prison. And now I know that it’s my own analogy, but her wall is decorated with flowers to disguise what it is, but also when somebody comes to tear it down they’re going to have to take all the vines and stuff down first. So somebody's got to go in there with some weed whackers.