Has the time come for my 92 year old Mother in Law?

Have you contacted a Hospice? They can help you with a lot of this.
If you see no facial signs of distress, grimacing furrowing of her brow, clenching of facial features then I would suspect that she is in no pain.
Use moist swabs to keep her mouth moist.
If she shows any indication of wanting to drink thicken water and place on a swab but do not put fluid in her mouth directly as she may choke.
There are indications that she is at End of Life.
There are breathing changes. There are changes to the skin coloring.
Talk to her, hold her hand.

It is difficult to watch, but if you are not seeing signs of pain I would encourage you not to think she has pain. They become ultimately just very tired, too tired, quite honestly, to even attempt to respond for you/to you. They eventually are happiest at rest and at peace. Try to accept that for her, and to sit with her. Read to her. If she is a lover of the word, read to her some of the most beautiful things in her bible. If not, read her some of the most beautiful words and poetry of man. Play the music she loved. Speak to her about your memories. Or simply sit and hold her hand. Let her rest. I hope that her passing is peaceful. Does the District Nurse call in hospice for you. I wish you all peace and love, and remember to take joy in the life she has lived, is still living.

It is so very hard to watch. But what a person's body that is near end of life does with nutrition is different than a healthy person. Forcing food either through NG tube or IV seldom if ever helps them, it just addresses our sense of helplessness. What was her overall condition before she stopped eating? I assume you already ruled out depression or other treatable reason for not eating. If so, then I think the best you can do is provide comfort, companionship and reassurance to her and know that despite how painful it is for you, you are easing her final days. My God bless you and your husband for the care you provide her.