My dad (90) has no clue his step son/sons-in-law have closed his bank accounts and has lost his entire life savings. What can we do?

My dad revoked my sisters POA about 3 months ago on advice of stepsons saying she was stealing from him. We now have no idea what we can do to help him. We do not know if his bills are being paid, where is SSI and pension checks are going, all his bank accounts he had were closed and life savings 200,000. Can my sister and I become a guardianship of a person and property to help him and can my stepbrother talk dad into revolking that also? Our plan was, because he earned all that money himself, when the time comes neither myself or my sister would be able to take him in so we would hire someone to go in his home and take care of him so we know he will be taken care of well.