How many people are we talking about?
The sleeper room itself is a set price, regardless of the number of occupants. (A lot of people misread the sleeper fare as being for each occupant!)
Then, each person occupying the sleeper pays the base railfare, which is exactly equal to the cheapest coach seat that Amtrak sells.
So, unless you got coach seats at the lowest fare bucket, it would be more cost-advantageous for you to move another person into the sleeper (sounds like grandma is alone in there, and there are two beds), since that would make your total cost go down. (In other words, if you got coach seats at $100 each, but the base railfare is $75, moving another person from coach to sleeper would save you $25...and that person would also get free meals in the diner, since those are included in sleeping class!)