Out of all his siblings, Sanji has a better relationship with his elder sister, as while she did partake in laughing at him as a child, she only did so to avoid being bullied herself and tended to his wounds in secret. She felt enough empathy for him to free him from his imprisonment and even tearfully told him that he would meet people who would treat him better.
However, Sanji still showed disdain for Reiju when they reunited as adults due to her role in dragging him back to the family, and her attempts to show him the benefits of a royal life despite knowing what he went through as a child. While watching him duel their father, she would be amazed with the strength and durability Sanji has gained since their childhood. Following his defeat, she would tend to Sanji’s wounds while voicing her surprise that he was the previous weak child that she once knew.
Though Sanji did not think much of her, Reiju treated his wounds after his brothers beat him once again and even questioned his choice to return. After Reiju was wounded by Charlotte Pudding, Sanji felt obliged to tell his sister the truth about the Big Mom’s plan to assassinate the Vinsmokes. When Reiju told him to leave her and the family, Sanji was vocally against the idea as that meant she would die, showing that in spite of everything, he truly does care for his sister. In response, a touched Reiju expressed that she was proud of his empathy, telling him the truth of their mother’s death and alleviating his thoughts of guilt for her passing.
When Sanji decided to rescue her and his family to get rid of his childhood grudge against them, and when it briefly seemed like he would not succeed, Reiju again expressed her pride in him for his empathy. She later expressed a smile after Sanji verbally disowned Judge again after he questioned the latter’s reason of liberating them from Big Mom’s clutches. As the Vinsmokes fought off the Big Mom Pirates, Sanji and Luffy quickly came to Reiju’s aid when Big Mom focused her attacks on her.