The Reason Why I Became a Bystander Sister

My older brother has a very troublesome personality. He's the type who hides his dark nature and smiles anyway. I've noticed this, but my brother doesn't know that I've figured it out. That brother of mine is currently in love. This is the story of a younger sister who plays the role of an "older brother lover" to push off her brother—who has a dark side that's tedious to deal with seriously—onto his girlfriend, while supporting his romance. ...Or at least, that's how it should be, but recently, something seems off about his girlfriend's behavior? ※Currently on hiatus. Planning to resume at some point.