It may not be direct incest, but Hari's relationship itself with the brothers is incestuous. For starters, I think it is weird that she happens to look exactly like their dead sister. Besides - this story focuses on these 3 brothers accepting Hari as a sister, and then goes off to the brothers starting to like Hari romantically. The oldest bro's own words: "Of course, you're our sister". So it's a sibling bond -> romantic bond. Not to mention, again, Hari looks exactly like their deceased sister. Do they not get reminded of their blood-related sister every time they see a carbon copy of her? Moreover, they actually fall in love with the same girl that looks exactly like their blood-related sister? To me, that feels incestuous on top of the sibling relationship they're trying to build with her.
That aside, I'd like to point out the fact that Hari was emotionally and physically abused in her past life by the brothers. They isolated her, made her feel like shit, and even hit her. Really hard for me to understand how Hari can even confront them in this current life, but I'm not gonna lie this sort of "I love the guy who killed/abused me in my past life" trope is hella common in these types of manhwas, so it doesn't bug me that much