What is the opinion of Islamic law regarding a marriage that took place secretly with a marriage contract and the wife's guardian being her sister's husband, and what must the husband do in this situation?

This marriage is invalid because it lacks the most important pillar of marriage, which is the wali (guardian). It must be annulled unless a judge who does not stipulate the wali rules it to be valid. As for the mother, her consent is not a condition for the validity of the marriage. Her obedience is obligatory if she prevents marriage to a specific person for a valid reason, unless the daughter fears falling into sin, in which case there is no blame upon her for disobeying her. She must be dutiful and kind to her mother. One should not despair of theponse to prayer. Umar said: "I do not carry the burden of the response, but rather the burden of the supplication. For if I am inspired to supplicate, I know that the response is with it." And in the Hadith: "The supplication of one of you is answered as long as he does not hasten, saying, 'I have supplicated, but it has not been answered for me.'" So turn to Allah with acts of obedience and avoidance of sins, and supplicate frequently.