There is something powerful to the fact that the “I love you” of A Court of Silver Flames is not Cassian & instead is the Archeron sisters.
… That the first person who replies, "I love you too” to Nesta; a person who has never been able to say I love you, be allowed to love or be safe in that. Let alone have it reciprocated, without being immediately followed by their death (or destruction of some sort). And that that person is her sister; who loved her before & will love her after, who would fight to keep her safe & actually take care of her.
And Feyre finally gets to hear that from her big sister; from the family she thought she never had. It gives a full circle from how she left; feeling out of place among the human world, to end up in Prythian in the first place, she thought she was alone & left out to dry (or more likely freeze to death). To then in her almost end for the family she did find, she is saved by the sister she always had; the one who followed her to the edge of the wall, fought for her & spent days trying to rip into Prythian; to save her no matter the cost, and when the time came she “gave it all back” to pay it.
Feyre loved her, even when Nesta thought she wasn't worthy of it. Nesta loved her even when Feyre thought her sister hated her. And they both finally get to hear it, say it, see it, feel it, know it. They are worthy & they are loved regardless, they have an unconditional family of relatives & found loves. Their story enters a new world, one that is no longer alone. And there's something powerful in THAT as the choice, “I love you”.
+ We all know Cassian LOVES Nesta, even Nesta knows CASSIAN LOVES NESTA!!! It's been shown, thought, practically said (just got cut off)… it would’ve been nice, but it's already known. It didn't need assurance like they did.