【正确答案】Dear Tom,
I am terribly sorry to tell you that I am unable to attend your weeding ceremony next Thursday evening.
That is owing to the fact that 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 wedding personally. I have chosen a small gift for you and will send it by post to show my best wishes.