i find interest in dainrirs brotherly aspect of their relationship. i can't say anything complex for sure as a 100% canon because ... hoyo barely gives us anything !!
seeing a sinner in game at all ( aside from surtalogi who we got the abyss model of for like one quest ) plus learning their true dynamics with each other was probably the biggest lore drop we've gotten in five years
really genshin does not like giving lore that isn't focused on playables much. but from the few scraps they give me here's my interpretation that i think fleshes 'em out to my liking
rerir to me, is the younger one. exactly by 5 years. i think the age gap adds a lot to the guilt side of it, more particularly on dains side. imagine someone who you've seen as a little brother for a large amount of time, and given that age gap had probably raised to some limited extent, turns out to be such a different person then you've thought. such deep trauma you never noticed before until way too late
if you haven't noticed, i REALLY like making dain guilt ridden. it's his greatest trait. make him a sopping wet cat NOW
also on rerirs side it adds some flavor too, someone you look up to and consider dear that you're close to but. not close enough. and the reason why is too out of your control to really figure out how to handle until it's too much. i think many people forget rerir is in fact, a grooming victim. and this part of grooming affecting your other relationships isn't something i see mentioned much
first time they meet again and dain has to see someone he's viewed as "little sibling" become so distorted from what he knew, and rerirs only glimpse of him is when he's being sealed away to his prison again. they never did get the chance to reconnect like they probably wanted
aaaahhhh ໒꒰ྀི ´ ꒳ ` ྀི꒱ i love the weird sibling dynamics between characters. so good. so good