What’s wrong when both are okay with it lmao? The case for incest being mostly wrong and unacceptable is due to consent being murky due to nature of familial relations and often can come across as one side enforcing their will to the other (usually the older or male one) while the other due to their attachment impassively drags on or outright opposes it but keeps mum due to fear of social consequences.
This is not the case in ideal scenarios shown as such in this where both sides want to fuck each other dry.