First of all, I really like the premise,

However, I have to agree with the critiques above. Since the dying wish is what sets the story in motion, we should know what it is.

An example would be from a favorite movie of mine called “Stealing?Home” (Critics hated it but I thought it was great) In the movie the lead is giving the task of disposing of his friend’s ashes (She was actually his babysitter, who was a free spirit)

The dead friend doesn’t leave any instructions, the lead is just told, “You’ll know what to do.”
The movie is about the lead trying to figure out where she would want her ashes spread and the story is told through a series of flashbacks.
Each flashback is set off as he meets old acquaintances.

The point being, in your story, you could do something similar.

She is given a? dying task by her grandmother, and while trying to accomplish the task, the reader would be shown a series of flashbacks as to why the lead was estranged from her family.
What happened twenty years ago that caused the lead to become exiled from her family?

Just remember, the story we are told at the beginning would not be the real story.
At the end of the flashbacks, there would need to be a twist. What everyone thought happened didn’t

Say, the lead was babysitting her little brother (Twenty years ago)
He dies while she was on the phone with her boyfriend (Back then there were home phones with cords)
Everyone blames her for her brother’s death and she?is cast out from the family.
Now she is back for the funeral and persona non grata, however as she meets different family members while trying to accomplish her grandmothers dying wish we are given a series of flashback to the night of her brother’s death.

the series of flashbacks end with the audience discovering the death was actually someone else’s?fault, not the leads.

Anyway; just some thoughts