The answer is that your father will have to move, but not immediately. If your father’s wife passes away and she leaves the house to her children, they will become the owners and can ultimately evict your father. He can probably delay the process for some time since they may have to go through probate to establish their ownership, and many states give seniors a grace period to find a new place to live. In these situations, many spouses leave their houses in trust so the surviving spouse can live there either for a period of time or for the rest of their lives.
For more about trusts, click here.