I can't remember the manga really well.

Well, I think the girl's parents died and her relatives doesn't want to take her in (I don't know if this statement is true regarding her relatives because it has been a long time since I read this manga, sorry). Then a man who the relatives don't know, says he is her brother and so he took her in. He has a very successful job. She is always at home and he is at work. Well, the only secret she doesn't know about him is that he is not her real blood-related brother. She can tell he is lying when he said he is her brother because she can tell by looking at his aura or something like that. When someone is lying to her, she can tell.

Oh, I think I might know this one. Kimi ga Uso o Tsuita?