I have enjoyed many a sangcheng, but part of me thinks they might actually just think the other is an overdramatic asshole? JC appreciates a thorough revenge but frowns on JL endangerment and emotional trauma. Did the incest need to be revealed so JL had to watch his aunt die? Did he have to be in danger? JC thinks no! NHS looks his alive brother who loves him and his nephew and thinks he can cry more. JC thinks its a dick move to ruin your sect. They may or may not fuck about it but its there
Your brain is full of powerful and compelling thoughts, anon. Nie Huaisang definitely thinks Jiang Cheng is being over dramatic about Wei Wuxian on several fronts. Your brother, alive, in front of you, and he didn't even mean to kill anyone important? An embarrassment of riches. That being said, while I don't think Huaisang and Xichen were anywhere near as close as Wei Wuxian and Jiang Cheng, I do think Huaisang understands being deeply angry with someone because a perceived lack of affection for and failure to prioritize a person you both loved helped lead them to their death.
On Jiang Cheng's end I think it's mostly a question of how much Huaisang not really owing Jin Ling and Qin Su anything in particular matters to him. Obviously Jin Ling should never be exposed to serial killers and Jin Ling's aunt should be left in peace if possible, but Jiang Cheng is a person who is comfortable with the idea of having a small circle of people who matter more than everyone else. Had Jin Ling been harmed directly it would become a question of whether Huaisang could scheme his way into arranging for Lan Wangji to stab Jiang Cheng before Zidian winds up around his neck, but as it is I think the big hypothetical sore point is if and when some kind of ongoing relationship moves Jin Ling from "acceptable collateral damage" to "protect at all costs" for Huaisang. That moment when a small tendril of affection is rising within Jiang Cheng but signs point to him being incapable of inspiring similar feelings in other human beings becomes extra fraught in the context of his boyfriend maybe not even liking him enough to avoid getting his nephew killed, and I do think that would be a relationship killer.