I'm trying to write a story...
( core premise)
Rarity after getting news from her doctor finds despite all the bucks she's received, is not capable of conception...

One day while visiting her elder sister sweetie finds Rarity crying and sweetie being the sweet as sugar marshmallow wanted to know why? It’s then discovered by eavesdropping that her sister has been to the doctor and been told she was infertile whatever this infertile was, but she wasn't going to sit back and watch her sister cry she would find a cure no matter the cost!

Sweetie, thinking if normal medicine can't help maybe the magical kind could and goes to twilights library for some digging! Unaware that magic IVF had some interesting risks...

it had been a few days and her sister's mood hadn't gotten any better... there has to be something in here? the small marshmallow filly thought, as she tore another bookshelf asunder searching for something anything that could help her sister...  before long though she found something it was stuck between the pages of a book with a weird title foeto statim fertilitatis confused she pulled out the spell and smiled...  "yes!" her voice cracked with a squeak as he rolled the parchment up eager to surprise her sister with what she had found.... and before Spike could complain about the mess the filly was gone...

as the filly rushed to her sister boutique she could only giggle as she hoped what she found could help, not understanding much about babies as she rapped on her sister's door…

what I need help with is turning this mess into a proper story.

the idea is simple
phase one
Rarity goes to a doctor and finds out she's incapable of conception. then sweetie comes over for a visit and finds out by eavesdropping. Sweetie, thinking if normal medicine can't help maybe magical could and goes to twilight's library for some digging!

phase two
she finds an old piece of parchment in an old book with a weird tittle, thinking she's found the answer she takes the parchment and rushes to her sister.

phase three
Excited the energetic filly presents what she thinks is the answer surprising her sister both with the old spell and with the fact she knew. rarity forgiving her sisters strangers and invasion of a ladies personal matters takes look att he old piece of parchment finding it had a smudge on it but was intact enough she can make out the spell.

phase four-part one
After some hugs and preparations, sweetie watches as her sisters cast the spell both of them unaware of just how close it would bring them. (fetishes ensue)

phase five
sweetie now under the influence of the strange spell can't help but be attracted to her glowing sister... (hypnotized unbirthing ensues) Rarity exhorted collapses only to wake up with a round belly.

notes
The spell is a literal plot device! if this story took place is vorequestria I would just have Rarity call her sister over to her. Before either tricking her into it or just forcing her in... But this isn't voreqeustria plus that just doesn't sound like something Rarity would do. so I came up with this...