"When I grow up, I want a son first then a daughter. So my son would beat up any boy that makes my little girl cry - Mia James.
Certainly, emperor Rui wasn't thinking this when his last born at the time was a little princess. All he thought was how powerful would that offspring be so he can expand his power. Though it wasn't his intention his sons were indeed overprotective of Rui Li.
They might not show it all the time because she could protect herself but when they felt something was wrong they would step up and take care of her. For instance, right now they had just found out she was missing and instantly began to look for her.
Rui Li on the other hand didn't know that her absence had caused trouble. She was sitting opposite the handsome brother who was hugging his knees trying to get as far away from her as possible.