This is the father-daughter relationship to the cured vampire/human doppelgänger Stefan Salvatore and the tribrid Lillian Gilbert-Salvatore.
Stefan has always made sure that Lily, Jax, and Nina were always safe, loved, and had a happy childhood. He was very much happy when he learned that his then girlfriend was pregnant with not just one child, but three. He vowed himself that he'd make sure that they had a childhood that he and his own brother never got.
They are referred to as "Stillian" by fans.