OmNom brings up very good points, to which I want to add that it was a curious decision on the mangaka's part to jump around between different points in the past and present, often announcing it a few pages into the event. It made for a confusing read where I had to go back and reread some parts to make sure I'd understood them, which really shouldn't have been the case with such a simple plot.
Furthermore, the art hampered the storytelling more than aiding it, as it comes across more like an unfinished sketch rather than being purposefully messy. I also mistook Kiritani (Yukihiko's friend) for Tooru a few times, because he basically looks like Tooru with glasses.
It's...well, it's something that Yukihiko manages to fall for not one, but TWO family members. And I'm not sure what to do with that information, to be honest.
In the end, I'm left with the impression that there's something missing. Alternating memories with the present didn't do the story many favors, and, despite sufficient time being allotted to everyone, they remain fairly shallow. I would've liked to see more interactions between Tooru and Yukihiko, told linearly, in a way that doesn't read like chasing obligatory plot twists.
I guess a 6 will suffice, since it does have potential but the execution doesn't live up to it. The clumsy writing and the art make it seem like a rough draft rather than a finished product.