I haven't read the chapter
i have an idea why she's dating this guy, she loved her brother, she thought loving her brother was wrong and she found herself this guy, then she found out that his brother wasn't her blood, and she can't leave this bastard because she's really scared.
Apparently you read poorly or didn't read at all, the main character is ready to do anything to help his sisters, if he finds out that this guy is bad, he will immediately help her.