How can I get my father-in-law to accept help in caring for his spouse?

My aged father-in-law is valiantly trying to care for his disabled spouse while stubbornly refusing the help of his children. It's taking a heavy toll but he continues to provide for their needs just barely well enough to stave off a mandatory intervention by health care workers or family. We realize he's clinging to his independence fiercely as their standard of living slowly deteriorates, yet he won't discuss it and runs his children away if they attempt to assist in any way. It's becoming very stressful and sad to even try to visit them. What should we do?