Dear Wang Fang,
Thank you so much for your kind invitation to spend the summer holidays with you. It was very good of you to invite me, but I'm sorry to say that I shall not be able to go. I will tell you why. First of all, my mother is seriously ill. And my father is always busy with his work. I shall have to look after my mother and do some housework.
Thank you again for your invitation.
Yours sincerely
Xiao Hong