Dear Sarah,
I am terribly sorry to tell you that I am unable to attend your birthday party next Thursday evening. That is owing to(因为) the fact that (同位语从句)my younger brother suddenly fell ill and was taken to a hospital this morning. I have to go there immediately and take care of him. As told by the doctor in charge, it will take around five days for him to recover and I have asked my boss for a leave.
I really regret that I cannot go to celebrate your birthday personally and would miss the perfect chance of enjoying myself with all our old friends. I have chosen a small gift for you and will send it to you tomorrow to show my best wishes. Besides, please give my regards to our friends when you meet them at the party.
Cordially, (sincerely)
Li Hua