Is the marriage contract concluded between the husband and his wife's father Islamically valid if the father was deputized to propose to a girl, but he proposed to her sister instead, and the contract was concluded without the delegator's knowledge until after its completion?

The agent must adhere to the limits of the agency; otherwise, he would be considered an unauthorized intervener (fuduli). Scholars have differed regarding the validity of a marriage contracted by an unauthorized intervener; some have invalidated it, while others have affirmed its validity, contingent upon the approval of the person for whom the contract was made. The aforementioned contract is valid according to those who permit the contract of an unauthorized intervener, and since you acted upon its terms, you have ratified it. The majority of scholars affirm its validity, while the Shafi'i school holds that it is invalid.