I mean everybody's got a line that once its crossed, whatever the media is, kills your enjoyment. I guess my line is anything that is likely to cause significant irreversible harm to yourself or someone else. It feels like this manga is just dancing on that line, and I would prefer it take a few steps away from that line (if not turn around and walk away from it) rather than cross it.Unhealthy obsession is the spice that keeps relationships interesting.
And the most expedient way it seems to me, would be to sink the unhealthily obsessive incest ship, and fix the relationship so that the two of them can at least talk and act like normal sisters. Ad if the ship never sinks and this becomes some sort of crazy incesty polycule orgy, all right Im down for that too TBH.