Part of the process, is that the body gradually starts closing down the digestive system. All resources are diverted to the core physical functions of heart, brain and lungs. If she eats or drinks, it takes very limited resources away from the systems she needs to stay alive.
Stopping eating is a survival mechanism for her body. You have nothing to feel bad about. Putting it baldly, your Mum isn’t dying because she isn’t eating or drinking. She isn’t eating or drinking, because she’s dying.