Dear An,
          I'm really sorry for having to delay your staying with me next week. I know you want to meet me at my home very much. So do I. Unfortunately, I must change our plan. My grandmother has just had a heart attack. Now, she is in hospital and my family must be there with her. At this time, I will spend time on looking after my grandmother.
          I feel sad that I let you down and I hope you accept my apology. When my grandmother becomes better and comes back home, we will meet each other at my home again, like our plan. Do you agree with me?
         
Your sincerely