She knew how insecure Flare had always been. He was always scared when he first came to their house.
He was scared to run around or play with Hao Yu. He was scared that they would hate him if he had done something wrong by playing around.
It took a long time for Yao Xinyu to let Flare warm up to their family.
After being in his shell for a long time, he finally came out and started allowing Hao Yuu to play with himself as he wished.
Then he slowly turned into a member of the family and started feeling responsible for many things.
One of the responsibilities he took upon himself is of course to take good care of his little sister, Hao Ran.
Just because he had successfully integrated himself into the family doesn't mean that he had lost all of his insecurities.
It is just that all those feelings were suppressed and kept a closed corner of his heart for now.