MC thinks way too much into his behaviour, that is how most people are. Depending on the situation and location people might seem to have different personalities in real life as well.
This is one series where I wish we had more chapters that continue after school, we also didn't even get to know Kuzumi's first name at all and only got 1 chapter from his perspective out of 50. FMC is also probably the most bearable tsundere(usually hate that character trope) ever, maybe...
I would say I still count this as incest since they have been raised as brother and sister since they were born, not to mention they have liked each other before they knew they were not blood related. Still I am interested to see how far the author is going to take the story.
I always wanted to see a couple continue after school but this is doing both at the same time, so I can't complain.
EDIT: For the people of culture check 160216 by the same artist it actually has a nice story and 'plot'.