I love thinking about these two sister cultures i love them i love them i love them theyre sisters
Sirens have a very training and job based society, you choose what youre gonna do and you stick with it pretty much, usually it is teens who take over menial jobs and take over childcare after its past the point of a siren's very fragile point in their lives, and it helps give the teen experience and increases the teen's social standing (which is also very important for sirens)
Mimir children are taught very early how to survive and what to avoid, theyre also very sneaky and good at hiding from predators, + theyre pretty durable and dont get injured easily, so mimir parents tend to just let their children run around (this was when siren and mimir civilization was a lot closer but since then only the siren's main city has survived) this leads some mimir children to be semi-adopted by schools, and the teen running the school is kinda just a babysitter
Sirens form very extreme bonds towards their children (typically. sorry cash. ..) and this makes family relationships incredibly strong, however, in adoption it can make the child's survival incredibly hard because the siren parents can sometimes even forget the adopted child's existence
however, Mimirs have a society where they take care of all children within a society, like any child can just walk into another random mimir's house and demand food and they will give food, and mimirs are typically very good parents, so they don't really mind having a small, very fragile thing that needs a lot of attention, esp since their other children dont need a lot of attention so they dont really have a workload :)
despite these differences, mimirs and sirens are very similar, they are both incredibly curious species that aren't discriminatory towards other species (sirens having extreme genetic diversity and mimirs being shapeshifters) this leads to them both being able to "infiltrate" each others very close societal bonds, Mimirs joining flocks and sometimes even being regarded highly in a Siren's circle, and Sirens becoming close friends to Mimirs due to the Siren's protective-ness
also fun thing to note, mimirs evolved (physically and culturally) as prey, and sirens evolved (physically and culturally) as predators shoutoutshoutout