John Gill Commentary Numbers 3:33

"Of Merari was the family of the Mahlites, and the family of the Mushites: these are the families of Merari." — Numbers 3:33 (ASV)

Of Merari [were] the family of the Mahlites, and the family of
the Mushites
So called from his two sons Mahli and Mushi, (Numbers 3:20) ;

these [are] the families of Merari ;
the youngest son of Levi, (Numbers 3:17) .