How do you say favorite younger sister in Hawaiian?

There isn't a single Hawaiian phrase that directly translates to "favorite younger sister." However, you can use a combination of words to express this sentiment:

* ʻO ia koʻu kaikuaʻana punahele (Oh ee-ah koh-oo-kah-ee-koo-ah-nah poo-nah-heh-leh) - This literally translates to "She is my favorite younger sister."

* ʻO ia koʻu kaikuaʻana i aloha nui ai (Oh ee-ah koh-oo-kah-ee-koo-ah-nah ee ah-loh-ha noo-ee ah-ee) - This translates to "She is my younger sister whom I love very much."

* ʻO ia kaʻu kaikuaʻana i makemake nui ai (Oh ee-ah kah-oo-kah-ee-koo-ah-nah ee mah-keh-mah-keh noo-ee ah-ee) - This translates to "She is my younger sister whom I like very much."

You could also use a more casual phrase like:

* ʻO ia koʻu kaikuaʻana punahele loa (Oh ee-ah koh-oo-kah-ee-koo-ah-nah poo-nah-heh-leh lo-ah) - This translates to "She is my absolute favorite younger sister."

Remember that Hawaiian is a nuanced language, and the best way to express your sentiment will depend on the specific context and your relationship with your sister.