I have heard about the rental girlfriend/boyfriend, and even the rental family member in the Japanese society, but this manga is one of a kind. It opened up more layers in a rental relationship.
At the beginning, the girl was renting the guy to grasp the kind oniichan she couldn't get/ she lost from her own oniichan. She realized very well that the rental oniichan isn't her real oniichan, and wanted to end the rental relationship. In the end, when she tried to mend the relationship, her biological oniichan pushed her away farther.
The interesting thing is, she tried to look for encouragement, family warmth, etc from her rental oniichan. At the same time, the rental oniichan was also trying to heal his pain and loneliness through the existence of a little sister, a family.
It definitely isn't a simple rental relationship.
I'm not sure why exactly the biological brother shoved away his little sister, even slapping her to bleeding. I think he was also lost and felt powerless as a brother, and eventually pushing all the blame to his little sister. Still not an excuse of course for his abusive attitude.
So far, I was still wondering if this was a story about trying to mend a broken relationship or getting away from a toxic/abusive relationship. It could be both or neither after all.
Anyway, I give this a 10. Still looking forward to the story.