Is sleeping 12-14 hours per day unusual for a 92 year old woman?

Our mother is being taken care of by one of our siblings. She is being given a prescribed low dose sleeping pill but also a benadryl each night and she sleeps from roughly 10 at night to 1 in the afternoon. If she gets up earlier, say 9 or 10, she is told by our sibling to go back to bed, its not time to get up. We are afraid that she is being abused by being forced to sleep, or at a minimum stay in her room, as a form of elderly care. We think that she is being manipulated to conforming to his lifestyle......sleeping late and going to work around noon to early afternoon. They don't eat dinner until 8-10 at night when he comes home. We have suggested getting help in daily to keep her company (we have another sibling that spends a few days a week there plus another family friend that visits so once she is up she does have company most days, along with our visits and occasional other family members visiting from out of the area. He doesn't want the invasion of outside help. Our main concern is that she is being put down so to speak in order to keep her under control. My brother is a responsible person. He's a biologist with a very good job and career. We just don't know if this is normal behavior, or whether we are right that the situation seems a little abnormal. Any thoughts?