This (joking) discussion about "is it really" actually brings up some good points that I'm sure the mangaka never intended. (If they did then we need a Physiological or Philosophical tag. This manga does deserve Reincarnation and Gender Swap tags if we're going to be serious about this though.)
Is it incest if the SOUL is related but the BODY is not? If it's only when the body is blood related then... If a baby was swapped at the hospital and later the adult daughter has sex with the father, is it incest? Does it matter if they KNOW about the swap or if they THINK they are committing incest?
Similar questions about Yuri and if one of the bodies has a dick. This can also delve into gender dysphoria (I think the term is).