How to Tell Them Apart

Looking at these pictures, can you tell them apart? All these were taken when they were around 18-21 months timeframe.

SY's pictures were taken when she's about 18-21 months, SJ's were taken during a studio photoshoot when she's ~21 months and Yao's photos were taken recently when she's 19months plus. I personally feel that Yao looks more like SY although there are some similarities to SJ.

Tips: 
SY's at the top left, SJ's at the top and bottom right and Yao's at the bottom left