I’m so sorry OP to read what you’re going through. Just so very traumatic for you and your family to experience. It’s hard to watch the death process, it’s not like films or tv drama at all; most people are unprepared as we are all sheltered so much from the reality of the death process. My heart goes out to you at this very difficult time. 
My parents used to own a nursing home.I was in my teens and early 20’s when I worked there for a number of years as a health care assistant.
Yes, patients do tend to sleep more and more as others have said, their body just shuts down over a period of days or weeks. They can look well, are always warm and seem quite normal really but have limited consciousness.
Once a patient is on that journey, it’s a process that you can see happening but it’s generally gradual. Some people just never open their eyes again, others have periods of lucidity- it just depends on the individual. Some days you’ll see little change.
Breathing becomes more shallow. Occasionally, people ( not all) get a death rattle, called cheyne-stoking which can be fairly upsetting to listen to, so prepare yourself for this if it happens. This is the abnormal pattern of breathing of apnea soon before death and it always takes place fairly close to the end. Skin usually becomes mottled, a sort of purple, brown, although still warm. Sometimes, you can see large areas of this discolouration on arms and legs especially.
The one thing I often noticed when a patient was close to death, was the smell. It’s actually not that unpleasant, but very distinctive. It can smell slightly sweet, it’s hard to describe. It usually happens 48-24 hrs before death. I remember one lady in particular, whose odour was incredibly strong in her final days. It’s all the chemical changes going on in the body. So you might notice this, but you might not. Hospitals smell strong anyway, so in that environment, it may not be obvious. Usually patients become incontinent, so may need to be changed by nurses if applicable. In the final days, usually just keeping patients comfortable and clean is all that’s necessary. So you might wish to wipe her brow or brush her hair.
Patients only go cold once death has taken them, so don’t expect her to feel cold. Although sometimes people can feel
rather clammy and slightly odd to the touch- almost slightly sweaty.
I used to massage the hands of those dying - just nice normal things you know? So I would recommend holding her hand as much as possible. I know that sounds so obvious, but sometimes we forget in our own distress. I used to massage the skin and nails with a nice handcream, chat to them, brush their hair, tell a joke, talk about nature, general news, a favourite book or the weather and smile at them. A calming, gentle approach I felt helped.
I remember seeing people become weaker and they may just suddenly die when you least expect it or they linger on. Sometimes they just fade away overnight or simply stop breathing after a period of seemingly more awareness. So it’s hard to know when the moment of death will happen, it’s quite sporadic, but many elderly people do tend to pass during the night when the body is at it’s lowest ebb.
I can only speak from my own experience. I looked after the elderly, so this is simply from my own perspective. It was often a relief to see patients out of suffering and the one thing I can say is that the body is literally a shell once you’re gone.
There was one patient who was in so much pain that I was literally crying in a heap after she passed, I was so relieved she was out of such awful suffering. I spent some time with her before the undertakers came and talked to her for a while. I could absolutely sense her standing right next to me for a few minutes, then she faded away. It was a such special moment. I still carry her in my heart to this day and often think about her. I knew she was at peace and free. I took so much comfort from that.
I hope that helps a little OP. Try not to be frightened, she will pass when it’s the right time. Just stay with her as you’re already doing and talk to her as much as you can when visiting. You’re absolutely doing the right thing.
Take good care of yourself and may your family be comforted and supported during this very difficult time. Sending you and your mother in law my thoughts and prayers