Dont know the reason of using the past perfect here ,what if said once she died?
This mother's last few weeks living with cancer were spent making a wish-list for her sons once( she had died)

Question

Answer

It's a defective sentence.  This would work:

"This mother's last few weeks living with cancer were spent making a wish-list for her sons to use once she had died."

Consider the meaning of "once" as "after".  The sentence indicates that there was a point in time in the past when her sons used (or could have used) the wish-list.  Their mother died before this point in time.