Sibilance is natural for almost every vocalist and is needed to understand speech.
Also, these vocal sounds contain a large proportion of high frequencies (thousands of Hertz). They originate in the mouth and by air passing around the teeth.
The relative proportions of these frequencies within a vocal recording are determined by many factors including the vocal style, the loudness of the performance, the frequency response of the microphone, the relative position of the microphone, and what type and amount of compression you’re dealing with.
Over the years I learned how to avoid excessive sibilance while recording vocals and even then sometimes is impossible to bypass this issue. Some vocalists have more sibilance than others and if you know them, you know how to respond the right way to his/her sibilance.
Here are some of my tools of the trade on how to avoid sibilance when recording vocals.