It's just people trying to cope with the sad backstory of MC's sister since they expect shit like rape to be used in a non-serious or exaggerated way in hentai games. Also because it's popular to use the "what if anything not real??" since for most people it's easier than coming up with an actual theory for any story.
yeah i understand why all of you would think like that, but my thinking come from all the times the sister and the maid have been together, first the maid has never talk directly to the sister, second when they were in the boat the maid just call the MC, she never mentioned the sister, and finally the way the maid use the sister to mantain MC away from the women in town, it feel like she doesnt really care about a little girl lost in the woods.
now, i had a feeling that the sister was a illusion in his flashback because he was really feverish in it.
but like all you said, it could go either way.
and the hole in my theory is that the MC is seeing the things his sister does because waky powers so, dont take it all serious.