What is the ruling on a girl undergoing surgery to restore her hymen just before her marriage, and what is the ruling on providing financial assistance for this?

It is not permissible for your cousin (maternal aunt's daughter) to undergo a hymen reconstruction surgery due to the uncovering of the severe 'awrah (intimate parts) without necessity, deception of the husband, and encouragement of deviance involved. It is also not permissible for you to give her money for this surgery, as it constitutes aiding in sin. The girl is not required to inform her future husband about the loss of her virginity if it occurred due to natural causes, such as a severe jump or a heavy menstrual period.