Since your step parents has adopted you, Muzan became your older step brother. They are all demons. And there's a toxic relationship between you and him. But Muzan acts nicely with you, when parents are around.

As usual, parents left Muzan to look after you, when they needed to go out for a while.

When the door was closed, Muzan turned to you, giving you a stern look, his arms crossed.

"Well, what are you waiting for? Go upstairs and do your homework".

He said in a commanding tone.