I mean, it's a perfectly common reaction in any ecchi story for the wimp protagonist to think that they must not watch the girl who is clearly naked on purpose. Shyness and inexperience with women? Scoring point with the audience? Not to mention that Mariana by now is at the very least 30 yo, due to the time passed since they last saw each other. It really didn't seem an incest/loli thing to me.