"Di mana adik kalian?"

Translation: Where is your younger sibling?

[Kauakolo]

God asked Cain,

[Mario_Fez]

Sure, if Cain is more than one person ;)

"Kalian" = "You", but it's always plural

[Kas334864]

Yeah, it would be more like Jacob asking his sons about Joseph.

[Jay.L2]

You are right!
Younger sister = adik perempuan / Younger brother = adik laki-laki / adik kalian (adik kamu sekalian) = your younger (brother or sister)