I need help finding a manga I forgot

Okay so I don't remember everything but of what I do was the manga was of a girl isekai (I think that's what's it called) it something prob game or novel. She had a "brother" that wasn't her brother and he found that out later in their life in the ?game? And then he killed her cause of that. And when she meet the main. heroine of the game/novel that heroine has her last life memerios but she's a bad ?heroine? And for some reason the game/novels trying to kill the game heroine. Oh and the "brother" is a leader of this dark thing. Okay that's all I remember sorry for. my English and poor explanation. It probably makes no sense.