My mother had carers at home, but not live-in as that was too expensive (she just had 5 or 6 hours a day). It worked very well for a while, they did everything for her - prepared her meals, fed her cat, did her housework, took her shopping, to the park, and to appointments. When she started needing help with washing and dressing they did that too. But when she needed supervision 24/7 I moved her to a care home.
I think all you can do is try it and see how you get on. How well it works depends on the agency and the carers they use, and whether your relative is prepared to co-operate with them. Are you sure she needs someone there full time - can she not be left alone at all?
I found my mother's care agency by googling 'dementia care at home' and then the location. The manager will assess your relative and talk to you to ascertain what care is required.