Sanji is hers.
Reige tried for years to love all her brothers. But Father claimed Ichiji as soon as he was born; Niji was so determined to carve out a personality for himself besides "the spare" that he never did have any use for the family; and Yonji was from the very first a little brat who resented his resemblance to Sanji.
But Sanji is hers.
Father claimed Ichiji and Niji; Mother trained Reige; but it is left almost entirely to Reige to train her baby brothers.
She decides to teach them instead. The distinction is very important to her; Mother trained Reige in subterfuge of all kinds, in sleights of hand and carefully disseminated information and subtle poisons and clever games with words. Reige learned how to imply things with just a glance, how to use her body as a distraction or a messenger of things unspoken. Mother specializes in precision work of all kinds; Mother is the best in the Germa 66 for any kind of intelligence work, for sabotage, for quiet assassinations. Mother trained Reige to be the Poison Pink, beautiful and deadly and so very, very clever.
But for all of her parents' power, Reige soon spots a weakness.
They're so cold, her parents. Father rules with intimidation, Mother with manipulation; neither of them seem to realize there is an easier and safer way to get better results from their subordinates, whether those subordinates are family or underlings.
Reige cultivates loyalty where she can and respect where she cannot. Unlike Father, Reige does not see kindness as inherently weak; unlike Mother, she does not enter every interaction with someone else looking for some kind of advantage. She tries to balance her agenda with her humanity, and it yields stunning results, even if she takes care to keep the extent of her success from most of her family.
From all of her family, actually, except Sanji.
In all of Germa 66, Sanji is the only one that Reige trusts. Sanji is a sweet boy, empathetic, intelligent, and eager to please; Reige enjoys teaching him. (He balances out Yonji, who never passes up an opportunity to argue with her and only concedes his point when Reige's thoroughly kicked his ass. Sanji tries to help her, but that only results in Reige having to teach them separately, since the resulting fight leaves her with a headache and an unacceptable level of property damage to explain to Mother.)
Sanji is hers, but Reige never thought he was serious about haring off in search of the All Blue.
It hurts for a long time. She understands why he left; hell, she empathizes. And while their brothers are dubious about his survival, Reige has enough faith in Sanji's strength and intelligence and, above all, his stubbornness to never doubt that he's alive.
When they receive word that the drawing on Black Leg Sanji's Wanted poster matches someone named Duval, Reige has an argument with Yonji that ends with Yonji twitching on the floor and frothing at the mouth for a solid ten minutes before Reige deigns to re- absorb the poison. (She is gracious enough to accept the booby trapped hallway to her quarters as fair play.) After the photo on a subsequent Wanted poster confirms for them that Black Leg Sanji is in fact Sanji Vinsmoke, Ichiji actually returns her smile.
Her Sanji is coming home.
"Rei- nee- sama," he breathes, using an almost reverent tone to enunciate the title only he ever used. It matches the awed expression on his face, the fierce scowl of earlier entirely melted away.
She touches down on the ball of one foot, opens her arms, smiles beatifically. "Sanji."
He rushes to her, and she recognizes the little boy he once was in that wide grin, the unreserved happiness in his visible eye, his laughter as he catches her up in his arms and spins with her. He puts her down only to tilt the bare portion of his forehead to touch hers, their hair screening enough of their faces to hinder anyone trying to read their lips.
"I missed you." Sanji whispers. "Only you."
"I know, baby brother. My best beloved. I missed you too."