I am so sorry for your loss.  You can not obatin a POA after a person passes away.  A POA is a document signed by the perosn for whom it is intended and it dies when th person dies anyway.  What you can do is to be appointed as the fiduciary (generally called the Personal Representative) of that person's estate if they dies without a Will.  If they had a Will then there should be a names "Executor" therein.  Good luck.