The Elder of Two Brothers Concluded the Contract, Though He Is Not the Guardian

Answer—and Allah grants success: The custom and common practice in these lands is to give precedence to the elder brother in such matters, and that is tantamount to permission. If the husband wishes to remove doubt entirely, let him request from the younger brother an approval of the contract; there is no need to renew the contract.
The scholars of the madhhab have said that approval (ijāzah) may be by word—such as: “I approve,” “I consent,” or the like—or by deed, such as demanding the dowry. Accordingly, the younger brother’s arranging of the wedding, inviting neighbors and relatives, attending the banquet, receiving the guests, and the like—constitute a de facto approval of the marriage contract.
Source: Min Thimār al-ʿIlm wa al-Ḥikmah vol.1