My mother-in-law is demanding and it overwhelms me. What are ways I can deal with this?

I wonder if you could start a support group in your area? I'll bet there are others in your shoes that would be relieved they're not alone! This care taking business can not be successful without support; a listening ear, task sharing, information sharing, etc.
my mamma goes to adult day care 4x a week, for the whole day, and I know all situations are not alike, but check that out in your community. If nothing else, they will have a host of information and insight. This journey is one-day-at-a-time.....but remember to take care of YOU as well!!!!!!

To the right here, is an orange title: Caregiver's Connection. Check it out if you haven't clicked on any of them. Also, the people here may answer you with some options you don't know of. I began taking off work to be home with my husband. He couldn't be left alone, and I did not know about these options of help. Whoever diagnosed your mother-in-law, the hospital or whatever should be consulted and see what support, providers, or agencies they could refer you to for help in taking care of her. You must also check out medicare, medicaid, nursing home options and which ones to put her on a waiting list. There will be a time you may consider these. There are patients rights in medicare and medicaid. Start now, cause there is alot! I found an agency, and and going to get the paper work started this Friday to see if he qualifies. Then maybe I can go back to work!