Storyline-wise it makes the most sense. Censorship dictates the default is "landlady" and "tenant" and "co-tenants" or "friends" or whatever between MC and Sky, but it's fairly clear they're mother and children with Klara and siblings. One could argue step-siblings, or foster mother and wards if you like and it would still make sense plot wise. Landlady and tenant is very amusing though, considering the reactions to the potential romantic interactions. Ch. 5 makes it pretty clear that the MC and Sky grew up in the same household though.