Does Islamic law guarantee rights to the husband against his wife and a strange man who spoke about matters concerning their marital relationship? And is the husband sinful if he accuses the sister of incitement to divorce and conveys the matter to her husband through official channels?

The default is to assume good faith in a Muslim woman, especially a wife. Accusation of betrayal is a serious matter that requires verification. If the husband ascertains that his wife is suspiciously conversing with a strange man, he must remind her of Allah and repentance. If she repents, then all is well and good. But if she insists, there is no good in keeping her. Rather, he must divorce her, based on the Prophet's (peace be upon him) saying: "Three types of people Allah عز وجل will not look at on the Day of Resurrection: the one who disobeys his parents, the woman who imitates men, and the Dayyouth (cuckold)." In another narration: "Three types of people Allah has forbidden Paradise for: the habitual drunkard, the one who disobeys his parents, and the one who tolerates evil within his family." The Dayyouth is he who has no ghira (protective jealousy) over his family. What the man and the wife's sister did is called "takhbib," which is corrupting a woman against her husband, and it is a grave sin and an act of devils, based on the Prophet's (peace be upon him) saying: "He is not of us who incites a woman against her husband." The punishment for the one who commits takhbib is a discretionary punishment determined by the Sharia judge, and he is imprisoned until he repents. The husband has the right to report the matter to the official authorities and the judiciary to curb their evil. The wife should first forbid her from this act and explain to her the danger of her relationships with strange men. If she responds, she should not reveal her secret. If she insists, then inform whoever can restrain her from that.