Older brother becomes little sister

My last idea that was loosely inspired by an anime has got me a few really good stories, so I thought I would try something inspired by another.  Note:  While the pair of sisters will be major characters I can go either way on it turning incestuous, there could be another girl as my character’s love interest.

The idea is a brother and sister.  We’re always close as kids, but between getting older and differences between boys and girls, separate friends and other things, they have gradually grown more distant.  Neither really wanted it, but after so long neither had an idea of how to fix it.

Then one New Year, they ended up going to the shrine together, still awkward, not much said.  But when they made their wish, it was for the same thing, to find some way to get closer again.  Little did they know they would get their wish, but in a way neither expected.

The next morning the brother would wake up as a girl and in a younger body.  The sister had in the 15-16 area, the brother had been 17-18, but now 13-14.  After initial shock, she reluctantly goes to her sister for help/advice.

That’s the set up.  I do want this to be a slow build up, an at least somewhat more realistic adjustment to the character becoming a girl.  Focusing on every detail that we are comfortable with, no matter awkward for the characters, I’m willing to go deep in that side of the story.  There will eventually be sex in this story but a rather slow build, first focusing on the changes to his/her life and their new sibling relationship.

We could go the incestuous route, though if we do those feelings will develop slowly.  Alternatively she could go for another girl, maybe a new classmate, or one of her sister’s friends, or something else.  At least for a quite awhile, no one outside of the family will be aware of the change or who my character used to be.

I do want a woman as a partner here, partially out of preference for that in all FxF stories, but also want someone with first hand experience with what her character will be teaching her new sister.