No, it is completely inappropriate. Watching someone undress without their consent is a serious invasion of privacy, especially when it's a family member. Such a topic involves unethical and improper behavior.
No, it is not ethical at all. Watching someone undress without their consent, especially a family member like a cousin, is a serious invasion of privacy. It violates basic moral and ethical boundaries and can cause a great deal of harm and discomfort to the person being watched.
Since we don't have much information from just this title, it might be a fictional story where the niece is involved in something that is considered unacceptable in the story's setting. Maybe it has to do with family secrets or improper behavior within the family unit.
Telling such a story goes against basic moral and ethical values. Everyone has the right to privacy, and a wife's privacy in this regard should be protected at all costs. It also shows a lack of respect for the relationship between husband and wife.
To prevent accidentally seeing your niece undress, always knock on the door before entering her room. Make sure she has proper privacy when she is changing, for example, by providing a lock on her bedroom or bathroom door if possible. Also, be aware of her routine, like when she usually gets dressed or takes a bath, and avoid being in those areas during those times.
One possibility could be that the niece discovers a family secret that she's not supposed to know, like a past scandal involving her parents or other relatives.
Immediately look away and leave the area as discreetly as possible. Then, try to forget about it and never mention it to anyone. It was an accident, and respecting her privacy is crucial.