Answer:
How are you? How are you preparing for your party? I am writing to inform you that I will be unable to attend your birthday party, as my mother is very ill and as per doctors advice, somebody needs to take care of her. Since my elder brothers and sisters are away from home, I am the only one left to look after her.
I plan to send you your birthday gift through your cousin Kannan, as he stays very close to our house. I hope you understand my situation. Here’s wishing you a lot of fun during your celebration with your friends.