Assalaamu alaykum. I am from Bangladesh. In my county, most of the people cannot afford an air conditioning system at home. So we need to keep the windows open, especially in the summer. Now, a woman does not wear the hijab in her home, but as the window is open, men from other houses can see her without her hijab. My question is, will she be sinful for this?
My second question is: how much of the hijab does a woman need to observe when a non-Mahram (permanently unmarriageable) man comes to visit her house and she needs to pass in front of him?
Third question: how much of the hijab does a woman need to observe in front of her close non-Mahram relatives, like the male cousins, when she is in her own home?
May Allah reward you.

All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger.

A Muslim woman is obliged to cover her body from the eyes of non-Mahram men, whether she is in her own home or outside. If a woman is not safe from non-Mahram men seeing her through the windows, then she is not allowed to appear without hijaab, otherwise she is sinful. She has either to wear her hijaab or close the windows or put curtains that prevent non-Mahram men from looking at her.

As for the limits of the hijaab that a woman wears in her own home in front of non-Mahram men, whether they are relatives or others, as long as they are non-Mahram, then she must cover her entire body, with a difference of opinion among the scholars whether or not she has to cover her face and hands.

For more benefit, please refer to fatwas 81554, 90268and 84729.

Allah knows best.