Leave as per eligibility can be considered on pro-rata basis. Especially when it's required on medical grounds supported by MC it's difficult to deny. Moreover social benefit measures under labour laws are minimum every employer should ensure and there is no bar in laws if much more is extended. Nothing illegal as you are proposing. It's 6 + 6 by the end of Dec. which shall be the balance.
2nd March 2019 From India, Bangalore