I have three brothers.
My brother, who lives in Sidney, came to see me last month. (non-defining)
My brother who lives in Sidney came to see me last month. (defining)
(mi hermano me visitó el mes pasado. Como tengo más de un hermano, debo especificar de qué hermano estoy hablando).
I have one sister.
My sister who is 25 years old spent her holiday in France. (defining)
My sister, who is 25 years old, spent her holiday in France. (non-defining)
(Mi hermana pasó sus vacaciones en Francia. Como sólo tengo una hermana, no necesito definir de qué hermana estoy hablando).
Tamara has two cats. Both of them are black.
Tamara's two cats which can play outside are black. (defining)
Tamara's two cats, which can play outside, are black. (non-defining)
(Los dos gatos de Tamara son negros. Como Tamara tiene sólo dos gatos, no necesito definir de qué gatos estoy hablando).
Kevin has four cats. Two of them are black.
Kevin's two cats, which are black, can play outside. (non-defining)
Kevin's two cats which are black can play outside. (defining)
(como Kevin tiene cuatro gatos, debo definir a qué dos gatos me estoy refiriendo).