Need japanese translation please?

Question:

I have a japanese friend and she´s turning 18 next weekend... Now she lives here on the us (perfect english) and she´s a japanese survivor...Came here on time. Also, she´s an anime otaku. So we´re doing a big surprise party for her. And we´re older than her and she´s petite and cute-looking so we call her our little sister (imouto?) Her name is Rina. So we´d like something funny and kawaii like ¨Happy 18, Rina-chan. I guess we can´t call you imouto anymore :)¨ How can I say that phrase? please in both kanji AND romaji if possible.
Arigato

Answer:

Hi, I'm Japanese.

Yae, "little sister" is "imouto"

"Happy 18, Rina-chan. I guess we SHOULDN´T call you imouto anymore"

translate into Japanese,

"リナちゃん、18歳の誕生日おめでとう。
もう妹とは呼ばない方がいいね。"

"Rina-chan, juuhassai no tanjoubi omedetou.
mou imouto towa yobanai houga iine"