Is it permissible to write a medical prescription in my name to benefit from the reimbursement of my sister's medication expenses, knowing that I was the one who paid for the medication for her?

If the prescription booklet for purchasing medicine is from commercial insurance, which is prohibited, then it is not permissible to benefit from it except for what was paid by the insured or the employer. It is permissible to use stratagem to obtain the entitlement. However, if it is from cooperative insurance, or if it is a gift or a donation, then the conditions set for benefiting from it must be adhered to, due to the Almighty's saying: "O you who have believed, fulfill [all] contracts," and due to the Prophet’s (peace be upon him) saying: "Muslims are bound by their conditions," and because violating them involves deceit and lying, which are forbidden in Sharia.