These can follow the original or brotherhood storyline
Twin of a hawk
This is the story of the twin sister of lieutenant Hawkeye a state Alchemist that is trying to find a way to change the past but instead of opening an alchemical portal to the past it opens to another world, she then falls the the air and lands on the ship of the world's greatest swordsman. This is a love story between the OC and Mihawk, crossover between Fullmetal Alchemist and One Piece
Sister of an Ishvalan
Once a generation their is an Ishvalan born to one family that is different than the others it can be anything from hair and eyes color to skin tone but this particular child is the most different she was born blind and with a weak constitution and so she doesn't most of her childhood indoors or at a hospital which is where she was on the day of the Ishvalan war when the way broke out there hospital staff didn't know what to do with her keep her there or cast her out since she was sick and blind but when the military showed up demanding the girl the nurse in charge of her smuggled her out to safety. She's a few years younger than Scar, this is an OC and Havock love story
Sister of the flame
Ryna is the twin sister of Roy Mustang. He is overprotective of her and even though she can take care of herself he won't let her go anywhere alone. When Scar shows up attacking state Alchemists and mistakes her for her brother and she almost dies she realizes that there are somethings that she can't do but when her brother shows up on the scene Scar takes her with him. This is a Scar love story
Twin from Xing
This is the story of Ling's twin sister. When Ling leaves Xing and goes to Ameastria she followed him but she loses sight of him when they arrived, so she went to Central where she runs into Colonel Roy mustang and is unintentionally dragged into his scheme to lure out the homunculus using barry the chopper afterwards, she heads North but she collapses in the snow and it's later found by Major Myles. This is an OC and and Myles love story
Older sister of Fullmetal
This is an OC and Ling love story
Sister of the Knight
This an OC and Fury love story