Shelley, it's not at all unrealistic that your mother might just fall asleep and not wake up. How is her cognition? Does she have an appetite, but simply can't swallow well, or has her appetite diminished so she really doesn't want much? Diminishing appetite is natural part of the dying process, and is usually indicative that the body is starting to shut down. As it does, the blood chemistry changes, which, among other things, will begin to affect her cognition. This is perfectly normal. It often happens that the decline is gradual, and that the patient begins sleeping more and more. Eventually, the patient falls asleep for the last time.
Your mother's weight is very low, and my guess is that she is near the end, especially if her appetite is very low. A common phenomenon very near the end is a spurt of energy and renewed appetite, but this is very transient. There is much hospice literature which gives a lot of information of what to expect throughout the dying process. Your hospice will probably give you some booklets about it. You can also find it online by searching such things as "dying process", or similar phrases.
I wish you much strength and comfort as you care for your precious mum.
-Phil