Louise, another who has been where you are.
Of course the other relatives are enjoying their wonderful lives. You are taking all the responsibility and doing all the work. You must change this or you will become very seriously depressed.
First stop your GP, tell them what is going on and the bad affect on your health. Tell them you need proper help. Then your Nan's GP - if it isn't the same person. Make sure they know exactly what your Nan's condition is and that she needs residential care.
SS will let you battle on until you go under. I'm lucky in that I had my husband to support me and say this is too much, or I'd have gone on and on til I crashed. SS must know you can do no more and Nan needs residential care. It sounds as though her dementia has really advanced and she needs expert care.
My Dad eventually needed residential care, I looked after him as long as I could, but it became too much. He wouldn't have wanted me to sacrifice my life and health to him. Your Nan probably wouldn't either. Get looking at Care Homes, I was so happy with the one that looked after Dad.
Age Concern has useful info. I too wonder if there are savings these other relatives want preserved for themselves.
You've given care and love to your Nan, now give it to yourself. It doesn't sound like anyone else will, so look after your own health and well-being. I know it's not easy when you haven't for so long.
I so wish I could give these to you in person.