Seems the anime and this manga are doing a different route haven't played the VN but this seems to be the loli or sister route while the anime seems to be nanase one as it focuses very hard on her while this one the not-a-loli.
I guess we had different ideas of normal. I haven't played the game so he might actually fuck his sister as well but what I was thinking of was sharing where the two don't touch each other. That's rare because normally every girl is getting dicked.