Of course, Mokou and Kaguya are related, but even Mokou and Keine. Mokou seems to be in good term with the rest of the Eientei's resident and since many humans were helped by Eirin's medical skill or escorted by Tewi outside the forest, I think even Keine is surely grateful to them and in good relationship too. So I somewhat consider both as part of the Eientei's group even if they aren't actually living in there.