Well, I have the sharing turned off, so in principle I won't even know when it can happen. For my part, if it's what you like and it exists, then better for you, since it's optional content (just like femdom and maledom), I don't see what problem there could be.
I like having the sharing option deactivated because that way I get the impression that they don't even consider it and I prefer that to having to say no every time and getting the impression that they consider it a viable option.
I guess that in general, those who don't like NTR or Sharing would prefer those options to be hidden and ineligible during their gameplay. Although I don't think it's something strictly necessary, just a preference that is more suited, I think, to their tastes, or at least that's the way that it's in my case.
Regarding the MC's relationship with his sister (and Hero - sister's alter ego), it may or may not be a problem depending on how it develops. For my part I will wait to see which way it takes before deciding if I like it or not.
I actually think that to eliminate headaches, it's best to include thoughts from the sister that help the player to consider that, even though she may not be fully aware of it at first, the sister suspects, at least in her subconscious, that the MC and the hero can be the same person.