You successfully passed a job interview. You are expected to start on November 15, but you will not be available on that date.
Write a letter to your new boss, explaining your situation, expressing your concern and suggesting solutions.
Dear Mr. Smith,
I am writing to inform you that, unfortunately, I will not be available at work on the appointed date of November 15.
This is due to an urgent family situation. I have just received an email about my sister who has fallen ill and is at hospital at the moment. As you can understand, I need to be there for her to offer some financial aid and moral support. That is why I kindly request you to postpone the date of my joining.
I understand that this issue may cause a certain inconvenience to you. For this reason, I would like to apologize and assure you that after visiting my sister, I will come back as soon as possible though not later than on November 18.
To compensate my absence, I suggest that I could fulfill my duties ahead of time, if it is suitable for you.
I look forward to hearing from you.