How do you say happy birthday little sister in Japanese?

十六才のお誕生日おめでとうございます : 'juuroku sai no otanjoubi omedetou gozaimasu'.

In case of a friend you can drop the ございます (gozaimasu) at the end.
十六才のお誕生日おめでとうございます : 'juuroku sai no otanjoubi omedetou gozaimasu'.

In case of a friend you can drop the ございます (gozaimasu) at the end.