My mother-in-law is exhausted from caring for her husband full time. How can we help her?

Time to have the loving talk. Are you able to give her a break? Maybe you and your husband can get care and household help set up?It is extremely difficult at first, when they are in denial and resistant. Also, go to needacaregiver.com and find a couple of them to come in and help. It's time. Yes, it's very hard:) We're here for you!

Thanks! it's my mother in-law. She cares for her husband 24/7 and fall asleep at the wheel of her car. She totaled a car last week (went into a ravine) and denies that there is a problem with her being the sole caregiver at 80! My father in-law weighs twice as much as she does, and needs ALL his needs to be attended, toiletting, feeding, bathing, affection. My husband is trying to figure out how to help his mother see reason. It's very hard.

Yes, that sounds familiar. You will find many here.
How can we help?
Is it dementia? Get a good geriatric psychiatrist to evaluate and adjust meds. If 'wife' and 'caregiver' are same person, please get another person to share the load.
Read: "the 12 resolutions you can keep" on this site. It's a start. Good Luck.