* Tui: This is the most common and general term for "sister". It can be used for any sister, regardless of age or birth order.
* Tuahine: This term is more specific and refers to a younger sister.
Therefore, to be precise:
* Older sister: You would still use Tui.
* Younger sister: You would use Tuahine.
It's worth noting that Māori language is quite complex, and the nuances of these words can vary depending on the region and the speaker's specific dialect.