Mom has dementia and still lives in her own home but keeps asking my sister who cares for her, when she will be going home. What do we say?

Whenever my Mom asks about going home I tell her it will be tomorrow, because I know my Mom has no short term memory, and "home" is the dairy farm that she grew up in another State. I know it is the farm as she frequently refers about the cattle. None of the other homes she had had any type of livestock.

Your idea to redirect your mom and tell her that she'll be going home tomorrow is a good solution. Keeping your mom calm in the moment is the most important thing. Insisting that she is already home will likely agitate her more. And I agree that her idea of "home" is some place else. Either the home in which she grew up or another place with happy memories for your mom. Bless her heart.