On-line Te Reo Māori Dictionary

Ko Joseph te tuakana o Isaac.
Joseph is the elder sibling (of the same gender) of Isaac.
Family relationships - older sibling of the same sex - tuakana

Ko Isaac te teina o Jo.
Isaac is the younger sibling (of the same gender) of Jo.
Family relationships - younger sibling of the same sex - teina

Ko Isaac te pōtiki o Phill.
Isaac is the youngest child of Phill.
Family relationships - youngest - pōtiki

Ko Isaac te tama a Phill rāua ko Angie.
Isaac is the son of Phill and Angie.
Talking about more than one person - ...rāua ko..., rātou ko...