Sanji has an immensely strained relationship with his family, the Vinsmoke Family, and has been estranged from them since he was 8.
Their royal pride and singular focus on strength and warfare conflict with his own dream and his chosen profession as a chef. His brothers used to brutally bully him because of “weakness” and his childhood tendency to cook for others, and they followed their father’s example in calling Sanji a disgrace to the family name.
At the age of 8, after being imprisoned by his father for 6 months, he chose to flee the Germa Kingdom despite being told that he would never survive alone on the high seas, indicating that he preferred to risk death rather than remaining with his family.
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.
After 13 years of estrangement, Reiju’s behavior towards Sanji was initially equivocal. At first, she sided with her family in viewing Sanji primarily as a means to achieve their ambitions. Sanji, in turn, treats her coldly in their first reunion since their childhood and rejected her attempts to re-establish their family ties.
However, after Sanji revealed to Reiju about Big Mom’s plot to assassinate the Vinsmoke Family during the wedding, Reiju believed him immediately stating he wouldn’t lie to her. She soon confided to him that she never believed that Sanji was a failure, since he grew to be the caring and compassionate individual their mother had always wanted him to be.
Reiju tearfully acknowledged his kindness and encouraged him to leave Whole Cake Island with the Straw Hats, even if it meant leaving her to die. Sanji, in turn, continually objected to the idea of allowing Reiju to die, revealing to a touched Reiju that he does care deeply for her.
Due to Luffy’s actions, Sanji was able to reach her and free her and the rest of his family. Reiju 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.
At Cacao Island, Reiju aided Sanji in escaping with his captain.