My Fellow Comrade (OC x Erwin Smith) Attack on Titan fanfic

Twins were born in the family of Karla and Grisha Yeager. The little sunshine Eren, who wants to reach freedom, and the ill red eye Kosara, who wishes to become the greatest mechanic in The Walls. The little ones become soldiers in Survey Corps after the death of their parents. Will Kosara ever become a skilled mechanic.