Okay I'm gonna post a comment in this chapter (again)
I have been (partially) in his position. I have a step big sister same age as me (but different month). I didnt realize until in middle school. Specially our prefix name is same. And realized why my big sister was in same grade with me when my friend's brother/sister were not. My parents always ignoring my question about that. Until when I was in highschool, my parents finally told about our family circumstances.
Now in the real life, it's weird enough to fall in love with your (step) sister especially I've lived for about 17 years in same roof. I never look my sister as potential love, neither is she.
But for the manga purpose, I didn't see any weird concept, because siscon and brocon are really common right 😂