Not that hard to read

Being with Mo YuMin for the past years when he visited YuYan in D country, Xiao FengYun's notice the looks between his brother and MouJin that took his attention. 

Knowing how dedicated was his big brother to take care of his siblings, Xiao FengYun also wanted for Mo YuMin to consider himself. 

Therefore, after his careful observation on how Mo YuMin turned a blind eye when MouJin helped YuYan to sneaked out, Xiao FengYun has an idea about his big brother's feelings. 

As for Mo YuMin, despite his awareness of MouJin's true nature, accompanied by her age, he secretly admires MouJin since he was a child. 

Therefore, Mo YuMin never looked at any woman nor being touched while he just keeps on hiding what he truly feels for MouJin all those years. 

Added by being with him since he was a child, his feeling only go deeper, but he doesn't know how to show it to MouJin with fear that she will decline him.