Is it considered a sin if the sister does not cover her face in front of her sister's husband, who stays overnight or spends the day at their house, and what is the solution to this issue?

It is obligatory to cover the face from the sister's husband, as he is a non-mahram (definition). It is forbidden to look at him or to be in seclusion with him. The Shari'ah has been strict regarding interactions with the relatives of the husband or wife due to the mixing and trust involved. Uqbah ibn Amir reported that the Messenger of Allah, peace and blessings be upon him, said: "Beware of entering upon women." A man from the Ansar said: "O Messenger of Allah, what about the hamw?" He said: "The hamw is death." (Narrated by Al-Bukhari and Muslim). The hamw is the husband's non-mahram relative. The hadith (definition) means that the fear of him is greater, and the temptation is more severe because he can reach the woman and be in seclusion with her without being questioned, unlike a stranger.