How come no Chinese related etymological word for Little Sister?

As I understand the casual terms for big brother, big sister and little brother have roots in Mandarin:

Big brother: Koko comes from Gege

Big Sister: Cici comes from Jiejie

Little brother: Dede comes from Didi

But I haven’t come across an equivalent for little sister being Maimai, little sisters are still called dek or dede just like little brothers

Any particular reasoning for this?