sooyoung because yoona and sooyoung are same age and sooyoung always take care of yoona.Futhermore,sooyoung and yoona like to eat so that's why they are called shikshin..
I think Yuri and Seohyun .. Yuri because they are room mates .. Seohyun because of their age .. Seohyun is the first maknae , Yoona the sekunde maknae ..