Disclaimer: J.K. Rowling's Patronus told me that she still owns Harry Potter.
About formatting:
For direct quotes I use italicized font.
The story went on like in canon….
Harry, Ron and Hermione were in the house of Aberforth Dumbledore, who tried to talk them into fleeing abroad.
Aberforth remained fixed in his chair, gazing at Harry with the eyes that were so extraordinary like his brother's.
Two memories flashed through his mind.
===O===
In the heat of three-way duel nobody was sure whose stray curse was flying towards Aberforth (it could've been even his own curse, ricocheted off a wall).
With a plaintive cry "No!" the little girl threw herself in front of her brother.
Then three boys stood horror-struck, looking at her dead body.
Without a word Gellert Grindelwald turned around and disapparated.
Albus and Aberforth watched, transfixed, how a scarlet fireball floated out of Ariana's mouth.
And how it slowly took shape of a magnificent magical bird.
(Yes, that's how phoenixes come into existence; when pure and innocent girl sacrifices herself to save someone, her soul may become a phoenix.)
Newborn phoenix started to sing. Its soft and eerie music seemed to transform the very fabric of existence. And in this transformed alter-reality thoughts and feelings were open and transparent.
From Aberforth came the silent plea, "Ariana, stay with me; I love you more than he does."
And from Albus – "Sister, I know that I don't deserve you but I need you more than he does."
And the wise and faithful phoenix had chosen the older and weaker brother.
===O===
There was a day when Supreme Mugwump, Grand Sorcerer, Chief Warlock, Headmaster of Hogwarts was laid to rest into a grand white tomb after a grand ceremony with grand speeches.
On the very same day an old unkempt barman was sitting alone in his old unkempt house.
Suddenly the portrait of young girl on the wall shone briefly.
And a half-transparent crimson bird floated out of the picture and landed on the table.
"Ariana," the old man spoke hoarsely, "You came back! How? How is it possible?"
The bird was silent, but its eyes seemed to penetrate the Aberforth's skull.
And then the thought that was not his arose in his mind,
- Brother, there will come a time when you seemingly have to choose between what is easy and what is right.
- Don't do what is easy, that comes from selfishness and selfishness corrupts.
- Don't do what is right, that comes from brain and brain is not infallible.
- DO WHAT IS GOOD, because that comes from heart.
And after this cryptic message – nothing.
For a while the phoenix and the wizard sat in silence, looking at each other.
They longed for each other but both knew that it was in vain.
Because phoenixes choose only once.
At last he cleared his throat, got to his feet, walked around the little table and approached the portrait of Ariana.
"You know what to do," he said.
And the story kept going on like in canon….
THE END
I know that in canon Fawkes is referred to as "he".
Well… let's say that although phoenixes are born from the souls of innocent girls, they acquire the gender of their chosen companions.
Aberforth choice explained:
Easy – just ignore whatever happens and let Death Eaters catch Harry and his friends.
Right – help Harry escape and flee (teenagers are not supposed to fight in adult wars).
Good… yes, that's what he actually did.