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