Don't do that. Book the stay as normal as if it were your mom going. Have her call the hotel and say that you will be arriving first and that she would like for you to be added to the reservation so that you can check in and get settle in before she arrives. They will add you to the reservation. Then you have rights to check in. When you do, get BOTH keys. Then they won't even know if she shows up or not later.
Good idea. I assume you don't think they'll follow up at check-out?