It's nice to see this one again. Although it got quite some backlash on batoto for bein somewhat similar to another manga I still liked it and I mean not like it bothers anybody if the x-th harem manga copies it's predecessors.
I become kinda suspicious about Erika. How can she just stay so calm? It wouldn't be strange if she didn't believe them or freaked out.
I think that male MC is always in a depressed state so much that those small peaks of happiness still don't overcome the looming sense of depression.
Just my thought but I'll still put it in spoiler:
One possible way I came up with an explanation for the remaining one year was that the Male MC was somehow ill and might have had only a short life ahead anyways. He, therefore, would have come to terms beforehand and try to put his sister on the "right way" by helping her enter a good school so that she can secure her future instead of telling her and f-ing up her entrance exam.
If not then the depression probably comes from a loved one's death, eg the father, and the hardships of poverty but that would not explain the 1 year left.