MSN Money has always had good articles on this topic.
Here's one on retiring early (although it seems to be aimed more at us younger folks).
Here's one on getting Social Security early.
Here's one on tapping the 401k early.
Here's a site on Oklahoma tax specifics for retirees, as well as other state info... I assume they'll be staying here if your FIL is in bad health and they're already established in the Tulsa area.
The big thing in all of this is that
every financial planner out there would dissuade anyone from retiring early except under the most dire circumstances, such as job loss or extreme medical hardship.
I think the best thing they can do is talk to friends their own age about their own retirements (or retirement plans), including recommendations on financial planners in the area, and other resources for retirees. If they are hurting for money, there's always reverse mortgages, or shared living expenses if they live with family or friends.
Now might also be a good time to decide how much help you and your spouse are willing and able to provide them, not just financially, but providing care and assistance in everyday living. Your kids might be a bit freaked out if they get home from school one day and find that their room is occupied by the grandparents and all of the grandparents' stuff and they have just been relegated to sharing room with a sibling!