Sinuses

It is sinuses, apparently the addition of ES or changing US to I is based on if the word is Latin based or not. Now to research all of that would probably take more time than memorizing which plural to use, so you may consider trying to memorize your -US ending words as you would have to any irregular plural nouns.

The plural of Sinus is: Sinuses.

The Correct plural, as this is latin, is Sinuus,the uu pronounced longer (look up u-declination)