God bless you both for your work of mercy - you may be the instrument of her salvation.
Death is the most important moment, returning to our Creator to stand in Judgment, and every step in these last moments should be carefully weighed.
It doesn't matter what the other heretical family members say/do unless they hold the purse strings. Otherwise, push ahead as hard as you can for her to have a holy death and funeral that will gain grace for her soul. That's all that's important.
Ditto, getting a priest there BEFORE she dies; however, a trad priest is best so that she receives actual extreme unction. "last rites' does not convey the same grace, nor exorcise. Even the holy water used by the novus ordo is blessed differently, no longer exorcising.
A trad priest can guide you regarding the wake, Requiem Mass, burial.
Please don't let her be deprived of this great grace, if you can. It's worth fighting for, even losing relationships over. I know firsthand.
As for you, be there as much as you can, praying the rosary with her, if she is able, and prayers for the dying.
Bring sacramentals to her.
And the Green Scapular - place it somewhere in her room where it won't be removed or disturbed, then recite the prayer for her each day.
Trust in Our Lord and His holy Mother - they do not fail, especially in these circuмstances.
Please, friend, do not be stressed. Don't you see? He is guiding you.