Sibilance is the sound someone makes when saying words that make the tongue push air through the front of the teeth, such as "zip" "jeep" "ship" and "chip". It can also refer to the non-word sounds that someone would make when getting someone's attention ("Ssst!") or telling one to be quiet ("shhhh!").
Fibilants are fricative consonants of medium to low amplitude and pitch, made by directing a stream of air passing through the teeth
It's is a language device, similar to sibilance, however it connotes a different meaning in a different way. Sibilance mostly focuses on the sound itself produced, whereas fibilance mostly focuses on the facial movements from the readers to convey emotion
Person 1 - "This author uses many 'f's' in this sentence, what could the language device be?"
Person 2 - "It's fibilance"