One possible version:
Dear Tom,
I was far too happy that we made an appointment to visit Beijing together next weekend. How I appreciate the opportunity to be your guide in Beijing! But I’m afraid we’ll have to make it another time, because I’ll be meeting my uncle at the airport. He is on business from Australia, and will be home for only two days. It’s a rare chance for our family to get reunited since he left home ten years ago. I hope you’ll forgive me for not keeping my word. If it’s convenient for you, we can make it the weekend after next, or you may make the choice, OK?
By the way, my family and I will be overjoyed to have your company when my uncle is home. If you have no plan, please let me know, and I’ll fetch you upon my uncle’s return.
Have a happy weekend.
Li Hua
略