Is the joking interaction that took place between the uncle and one of my siblings, which included the specification of a dowry (mahr) and deferred dowry (mu’akhkhar), considered a valid marriage? And is the subsequent marriage of the girl to another person considered void, knowing that the girl was not of marriageable age at the time of the joking and her opinion was not taken?

Jesting in a marriage contract is considered as serious as earnestness, according to the Prophet, peace and blessings be upon him: "Three things, their seriousness is serious, and their jesting is serious: marriage, divorce, and a return [to the marriage]." So, if the offer and acceptance are completed in the presence of two witnesses and the girl is specified, the contract is valid even if they were joking. The consent of a young girl is not a condition for the validity of the contract according to most scholars, and her father may marry her off without her permission. If the first contract is valid, the second contract becomes null and void. It is advisable to refer the matter to the Sharia (definition) court for consideration.