Never Give Up On You

Natalia Romanova a.k.a Natasha Romanoff has been an agent of SHIELD for years now. One of the best, and some say on of Nick Fury's most trusted aids, being run up only by Maria Hill and Phil Coulson. But even Fury doesn't know about the one secret she keeps close on her heart...
She's been looking for someone, for years, trying to find the one person she thinks she failed. Her brother Aleksandr Romanova.
Both recruited at the same time, they two were close, until Natasha's skills proved too good to waste, and his... Not enough.
For years the trail has gone cold and Natasha's all but given up on finding him again. When a freak sighting of a new HYDRA agent on a mission sends her fighting to find him and bring him home with her once again.