Learn Finnish: Lapsi istuu äidin ja isän välissä. - The child is sitting between mother and father.

They are in the genitive case because of the postposition välissä (between). In Finnish, most postpositions require the words in front of them to be in the genitive case. You can think of it as literally saying 'in the between of mother and of father'.

Why is äidin spelled with a 'd' when the basic word is äiti?

This is due to consonant gradation. Adding the genitive -n ending closes the final syllable, which forces the strong consonant t to soften into a weak d. The word isä doesn't have a changing consonant, so it just simply becomes isän.

I noticed välissä ends with -ssä (in). Does a postposition take case endings too?

Yes, many do! Välissä is formed from the noun väli (space, gap) and the inessive ending -ssä (in). Finnish often uses location endings on postpositions to show movement or stillness. Here, -ssä shows the child is stationary in the space between them.

Why is istuu translated as 'is sitting' instead of 'sits'?

Finnish doesn't have a separate continuous tense like English does. The single present tense verb istuu covers both 'sits' (as a habit) and 'is sitting' (right now). The translation 'is sitting' is used here because it sounds more natural in English for this situation.