Decline an invitation of a birthday party　　Dear Lisa,　　Thank you for inviting me to your birthday party.
I'd love to,but sorry,I can't,because my sister is ill and my mother has to work,so I must look after my little sister.
Although I can't go tomorrow,trust me I make sure that I will go to your birthday party next year.
I hope that you can have a great time tomorrow.
　　Yours,　　Name