The guy was just asking whether IF BY CHANCE nakaboso na? So it is a hypothetical question. From an academic point of view, he was trying to open a topic and subject the act to a discussion. What people here should do instead is to provide a point of view on the subject at hand. He was not even saying he is doing it or he will do it. What I see are all do-gooders or "holier than thou" people who perceive themselves as "normal" or "saints".
If by chance you see your sister nude or happen to open the door to her bedroom and find her nude, that is just normal. What you do is just apologize. But if you do that intentionally hoping to find her nude or in some compromising position, you should start asking yourself whether you have an incestuous desire on your sister. Then, we go to the moral plateau which is more a question of perception and hence, subjective. Different cultures have different perceptions. What may be "moral" under Philippines laws may just be a "practice" or a "normal" behavior in some other cultures. Hence, we can never be judgmental. Incestuous behavior are normal practices in some families or cultures and we can not use your own personal psyche or personal standards as the behavioral barometer for all cultures and people.
If anyone thinks that smelling a sister's underwear is "normal", that is his/her prerogative. Sometimes, it is also a matter of social economic norms. I know people in the exclusive villages who find "incestuous" acts as "normal" but may be considered a "perversion" by those in the lower economic class. Maybe, society can be a better served when everyone would just mind his/her own business and do whatever one feels doing so long as his/her act would never put anyone into a disadvantage.