Should one inform a brother of what his daughter saw his wife doing in a suspicious situation with another man in his home while he was traveling, and is the questioner sinful if he asks him to divorce her?

The basic principle is to assume the best and good intentions regarding Muslims. If there are indications that a person not accustomed to committing a reprehensible act has done so, then they should be covered and advised, for the Prophet, peace and blessings be upon him, said: "Whoever covers a Muslim, Allah will cover him on the Day of Judgment." If they do not desist, then it is permissible to inform about them. As for someone known for corruption, they should not be covered. This woman should be advised and her actions kept private if something suspicious emanates from her. If it recurs, her husband should be informed.