adik (Indonesian) comes from Malay adik, from Proto-Malayic adi-ʔ, from Proto-Malayo-Chamic adi-ʔ, from Proto-Malayo-Sumbawan wadi-ʔ, from Proto-Malayo-Polynesian huaji-q, from Proto-Austronesian *huaji, from Proto-Austronesian Suaji — sibling.
adik (Indonesian): sibling