You and your Dad are both caring people. I have been down the road - at least to an extent - that your Dad is going down.
I have an old thread posted here called "What to do?" It documents the last 7-months or so of my Mothers life while she was declining with Dementia.
My family is/was very small - no brothers or sisters ... and I have cancer. So my help was limited to seeing her every week for the last 5-years or so - and paying her bills for close to that timeframe.
But, her last years were in an excellent local Assisted Living Home for Dementia and Parkinsons. It was costly at about $6000. per month - but they were excellent. Nursing Homes - in my area were about $5000. per month and were not equipped well to handle Dementia or Alzheimers. And those that were approved for Medicaid were terrible places to send a loved one.
Before the Assisted Living, she was at her own home but with a 24/7 caregiver. That too is expensive. The group we used was about $5000. per month and they were toward the lower end of pricing.
She was fortunate to have savings that paid for the majority of those expenses for close to three years. Last Summer, as her savings were getting low, I put her house up for sale. It was sold and allowed her the available cash to live up to two more years in that Assisted Living Home.
Everthing is so expensive - for most people. And Dementia patients need the type of care that often cannot be found in a Nursing Home. Finances are difficult ..... and the typical Dementia patient unknowingly makes demands upon their caregivers that sometimes are difficult to live with.
I would highly suggest your Dad (and maybe you too) go to a counselor who works with caregivers. Both of you need re-assurance at times - and certainly need regular breaks. There are agencies of volunteers who may be able to help you with periodic breaks.
You may find a support group for caregivers - and I would highly suggest that too.
I did at one point, investigate government assistance - because my Father was in the armed forces. It is available for the spouse of a veteran - BUT .... only after the spouses total net worth is down to $30,000. including the value of a home. And then, the maximum I found was about $1000. per month ..... and you can't get into any kind of a retirement home for 3x that amount. So, that only helps if there is a big social security check and pension check coming in - besides the government assistance.
It is a dismal financial situation.
One other thought is hospice care. It is available thru medicare - at no cost to the patient. While it usually is short-term care - it is sometimes available for more han 6-months. It must be recommended by a doctor though.
Note - all of the above financial info is based on living in the U.S.!
I am so sorry both of you are going thru this ..... and realize it's doubley painful because you love grandma!
Peace and wellness
Rob & Gizmo