Go ahead and elaborate in my case.
Because I recognize this movie has its qualities.
Um, that I believe it does hold up well and the plot is good?
Sorry, since that's all you said, that's all I can respond to.
Sure, but the rest of the Sully family - to whom he is very much a brother - didn't say anything on this either. And to us as the audience it was obviously supposed to signal how ready she was to kill someone who was practically part of her own family for revenge.
Okay, there's basically two ways that we can go about this.
One, from a canonical perspective, Spider isn't part of the Sully family, there's absolutely no way you can claim otherwise. He was born c. 2154, and was taken in by the McCoskers. Yes, he spent a lot of time beyond Hell's Gate, with Jake's kids and so on, with varying degrees of reciprocity (Kiri is close to him, Neytiri despises him, Jake's sympathetic to him, Lo'ak and Neteyam are generally cool with him), but he wasn't a family member. Certainly not before the RDA's return and the main events of 'The High Ground.' To claim that Spider is part of the family is to either ignore canon or retcon it.
Second, even if we're confining this to the film and nothing else, Spider still isn't treated as a family member. Jake refers to Spider as "inseperable from our kids," not "one of our kids." He isn't even adopted in the same way Kiri is, he's just a child who spends a lot of time with them (he isn't in the family photo either). There's no point in the film where Jake calls him "son" or any of the Sullys a "brother." Jake leaves to protect "this family," but Spider's already been captured at this point, and while Jake's clearly sympathetic, he doesn't make any effort to save Spider, since his own family takes priority. No-one even really brings up Spider after the jungle firefight, and while Kiri is clearly the most distraught, she's still one kid out of four.
As for getting to the end, well, I really don't agree. Again, there's nothing in the film to indicate Spider's a 'son' to Neytiri, and her disdain, while not as extreme as we've seen earlier, was established within the first ten minutes. It doesn't make sense for her to kill "her own son" out of revenge for Neteyam's death, it DOES make sense for her to kill Spider as Quaritch's son, and I'd argue the dialogue backs this up ("a son for a son"). Quaritch has killed her son, now she'll kill his if he doesn't play ball. You can certainly interpret the line as Neytiri has lost a son (Neteyam) and gained a son (Spider), but there's not really much in the story to support this. Neytiri never shows any affection to Spider, Spider is clearly terrified of Neytiri as he watches her slaughter the RDA's goons, and while reconciliation could occur between them in the future, it definitely doesn't occur in the second film, and I doubt it'll occur at the start of the third. After all, Spider's now alone in the company of Neytiri who hates him, an entire clan who has reason to hate him, and sooner or later, everyone will realize that Quaritch is still alive, and that he's the reason why.