Ymir being bound to indivuduals with royal blood it's not something magical, it was psychological, she's a slave even in paths. She was a slave to the king and she believes she is a slave to his children too. Same reason why she sacrificed herself to save the king, she's a slave and has stockholm syndrome. But Chadren now convinced her that she's a human being.
BASED CHADREN, BASED CHAPTER, BASED RUMBLING