One of the uses of Ser is to express relationship between people:

Yo soy su papá.
I am their dad.

Ella es mi hija.
She is my daughter.

Te presento a mi esposa, Cathy y a mi hijo, Buzz.
This my wife, Cathy and my son, Buzz.

Soy el marido de Cathy.
I am Cathy’s husband.

 

Mis hijos, o sea, mi hijo y mi hija, son hermanos.
My children, that is to say, my son and my daughter, are siblings.

Cathy, mi esposa, es su mamá.
Cathy, my wife, is their mom.