OK! So here’s the little backstory within my “Hogwarts AU” of how the sisters came to have their animal companions…

Starting with Edith,
she decided to go “very traditional” in terms of who her companion
would be and chose an owl (barn owl, specifically).  Owls also represent
wisdom, which makes perfect sense for a Raveclaw. Edith received her
owl the day after her letter arrived from Hogwarts, a gift from her
parents.  Edith continued with “tradition” and chose to name her owl Rowena,
after one of the original founders.  While Edith didn’t know for
certain she was going to be sorted into Ravenclaw (there was a 50/50
chance she’d follow in the path of her sister, father, and grandparents and be sorted into Slytherin) she had a strong inkling Ravenclaw would
be the house for her.  Edith loves Rowena, and sometimes sees the owl as
her closest friend and confidant. And just as Edith and Mary butt
heads, it isn’t too surprising that Rowena has been known to fly at
Mary’s dog with her talons bared…

Mary is very proud of
her Slytherin heritage, continuing the family tradition and following in
the footsteps of many of her Crawley predecessors. And just has her
father kept (and keeps) dogs, Mary also follows that Crawley/Slytherin
tradition and has a black half-dog, half-wolf hybrid whom she named
after the infamous Greek “hound of the underworld”, Cerberus.
Although unlike that hellish hound, Cerberus has only ONE head, and
despite his fierce looks and deep growl when strangers approach, is
truly a sweetheart when in the company of his mistress.  Cerberus came
to the Crawley family long before Mary received her letter; Mary always
believed she would be a Slytherin, and had decided when she was a small
child that she wanted a dog. She loves Cerberus, who loyally sleeps on
the floor next to her bed, keeping watch and growling at any passer-by
(save Sybil, who has somehow managed to charm the dog and turn him into a
big puppy, much to Mary’s annoyance). Whatever Mary does or wherever
she goes, Cerberus isn’t far behind…

Sybil knew she
wasn’t a Slytherin; her ambition was nowhere near the level of her
sister’s or grandmother’s. She loves to read the same way Edith does,
but she doubted she would find herself sorted into Ravenclaw. In fact,
everyone assumed Sybil would be sorted into Gryffindor, so it came as
quite the surprise to the Crawley family when she donned Hufflepuff
black and gold (She’s the only Crawley to ever be sorted into
Hufflepuff!) With time, everyone saw that it made sense, given Sybil’s
inclusive nature and lack of fear when it comes to hard work. Proud of
her independent spirit, Sybil thought it only made sense to have a cat
as her companion, a cat who found her, crawling into the Hufflepuff common room to escape a storm and curling up under Sybil’s sheets. Sybil named the kitten Christabel after
the suffragette Christabel Pankhurst. Christabel is very loyal to
Sybil, curling up in her lap when she’s sitting, and on her pillow when
she sleeps, but she’s not afraid to show affection for others, including
Mary’s dog Cerberus…

And finally, Rose is a Gryffindor
through and through, brave and fearless, but also a little bit reckless.
Her bravery was discovered when a boy put a toad in Rose’s hair.
However, Rose showed no fear or disgust, and lovingly adopted the toad,
whom she named Hercules, vowing to take him with her to
Hogwarts (much to her mother’s relief). Rose carries Hercules in her
pocket and never travels anywhere without him, even to the dinner table,
much to her mother’s horror. She finds him adorable, and often mimics
him when his throat expands revealing a large bubble before he croaks.  
Just as Rose can sometimes be a bit reckless, so too can Hercules, who
she has to keep a close watch on, especially in potions as he sometimes
shows an interest in jumping into her cauldron…

**images not mine