Karl and his little brother Thomas were taken into a foster home when their father died and mother was hospitalized! Sadly the system in their time is not super good and sometimes awful people use it to get more money from the system by fostering kids they then proceed to treat badly, and this is what initially happened to those two. However, Karl's teacher discovered what was going on and the Acker brothers were move to another foster family. Perhaps due to their first experience with a foster family or because I just suck as a writer, Karl and Thomas aren't super close with them? They don't open up to them easily but despite that it's super clear they all care for each other a lot and their foster parents are super sweet and loving people! They're already on the older side and they lost their own children so they are very fond of the two boys even though they're reserved, and I like to think with time they grow much closer. Both of the boys call them mama and papa though (different terms than what they use for their biological parents) so they definitely do accept them as their parents
Agnes and Cau were both adopted by a keykeeper named Agatha. Their relationship was a little weird as especially Agnes always saw Agatha more as a teacher than a mother? Cau also shares similar feelings about her but she is still their legal parent and their relationship has always been very warm!! In their main canon Agatha has already passed away though and Agnes took a one year off of work when this happened, as she needed time alone and spent it traveling around and meeting people who had known Agatha and visited places she had.
Mikael is eventually adopted by a young university student called Augustus! Initially he simply takes care of Mika since he's the only one who can communicate with him but honestly he really fast assumes the role of a father?? Mika is very clingy at first and hides under Augustus' jacket and in general depends on him a lot and in turn the man is super loving and surprisingly good with kids, although he definitely get help from everyone in the village too since they're all very fond of this strange, winged child.