@Eeveechi, she doesn't need to be related by blood to be Eugene's sister because for like 21 years (including her previous life) they grew up as siblings, so it is incest to be in a romantic relationship with someone you've seen as a sibling all these years.
I don't know whether It's me or the manga. but I find this quite boring and I can't bring myself to like the ml I'm not sure whether I'm just used to the Kuudere/Yandere type ml. 🤔 😒
Well, that was straightforward.... https://sdk.bitmoji.com/render/panel/93b73ae4-3be1-423f-a3b7-4cedb431e7a9-e525aba8-a312-42fd-a71d-24b0c887876c-v1.png?transparent=1&palette=1