What is the ruling on a strange man spending the night with his wife in a house where there are only women, knowing that this led to the violation of privacy, women being seen without hijab, and the bathroom door being opened without permission, and that the questioner's mother conceals her refusal to host them fearing for her relationship with her sister?

The husband of your maternal aunt is a stranger to you. It is not permissible for him to be in seclusion with any of you, nor to see her in her work clothes. Rather, you must cover yourselves from him. Seclusion is negated by the presence of another woman or a mahram (definition). Therefore, there is no harm in the maternal aunt's husband staying overnight in the house, provided that there is no fear of fitna. He must seek permission before entering upon the household, because seeking permission is obligatory for anyone whose awrah (private parts) should not be exposed, even for relatives like a mother or a sister. The obligation to seek permission from strange women is even stronger. If he seeks permission, his stay is permissible; otherwise, it is not permissible for you to gather in one house.