When a parent is aging or dying

Caring for parents is, in the canon, close to sacred — a family that honours its parents is said to have Brahmā living in it. Alongside that the Buddha asked everyone to reflect often that they will age, fall ill and die, and lose everyone and everything they love, so that when it arrives it is not a stranger. The suttas hold both: the duty and the truth. What is happening medically, and what to expect in the days ahead, is for the doctors or the hospice team to tell you; ask them as often as you need to.

What the suttas say

“Those who have a hundred loved ones, Visākhā, have a hundred sufferings. Those who have ninety loved ones, or eighty, seventy, sixty, fifty, forty, thirty, twenty, ten, nine, eight, seven, six, five, four, three, two, or one loved one have one suffering. Those who have no loved ones have no suffering. They are free of sorrow, stains, and anguish I say.”
Visākhā's grandchild — the Buddha's plain acknowledgement that love and grief come together.
Where this stops: Not a counsel to love less; a description of the terms on which we love.
Ud 8.8:5.1–5.20 · With Visākhā
Mendicants, a woman or a man, a layperson or a renunciate should often review these five subjects. What five? ‘I am liable to grow old, I am not exempt from old age.’ A woman or a man, a layperson or a renunciate should often review this. ‘I am liable to get sick, I am not exempt from sickness.’ … ‘I am liable to die, I am not exempt from death.’ … ‘I must be parted and separated from all I hold dear and beloved.’ … ‘I am the owner of my deeds and heir to my deeds. Deeds are my womb, my relative, and my refuge. I shall be the heir of whatever deeds I do, whether good or bad.’ A woman or a man, a layperson or a renunciate should often review this.
The reflection the Buddha asked everyone, monastic or lay, to make often.
Where this stops: Never preached at someone frightened or grieving; owning one's deeds is never a reason to call an illness or a loss someone's kamma.
AN 5.57:1.1–1.9 · Subjects for Regular Reviewing
Mendicants, a family where the children honor their parents in their home is said to live with divinity. A family where the children honor their parents in their home is said to live with the first tutors. A family where the children honor their parents in their home is said to live with those worthy of offerings dedicated to the gods. ‘Divinity’ is a term for your parents. ‘First tutors’ is a term for your parents.
On caring for parents.
Where this stops: It assumes parents who raised and cared for their children; never use it to press anyone back into contact, or into care they cannot give.
AN 3.31:1.1–1.5 · With Divinity
Ānanda, did I not prepare for this when I explained that we must be parted and separated from all we hold dear and beloved? How could it possibly be so that what is born, created, conditioned, and liable to wear out should not wear out? That is not possible.
Ānanda's grief at Sāriputta's death, and the Buddha's answer.
Where this stops: Never offered to hurry grief, and never use its point about impermanence to tell someone their loss should hurt less.
SN 47.13:6.1–6.4 · With Cunda
Mendicants, there are five things that cannot be had by any ascetic or brahmin or god or Māra or divinity or by anyone in the world. What five? That someone liable to old age should not grow old. That someone liable to sickness should not get sick. … That someone liable to death should not die. … That someone liable to ending should not end. … That someone liable to perishing should not perish. …
That what ages might escape aging — the impossibility named without softening.
Where this stops: Never offered to hurry grief or to tell someone they should stop grieving.
AN 5.48:1.1–1.4 · Things That Cannot Be Had

Bring your own situation.