"Di mana adik kalian?"

Translation: Where is your younger sibling?

[Kauakolo]

God asked Cain,

[Jay.L2]

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

[AndreasRtz1]

Why sister, not brother?

sister = adik perembuan
brother = adik laki-laki