John Gill Commentary Psalms 50:20

"Thou sittest and speakest against thy brother; Thou slanderest thine own mother`s son." — Psalms 50:20 (ASV)

You sit
Either in the chair of Moses, or on the seat of judgment, in the great sanhedrim of the nation; or, as Aben Ezra paraphrases it, "in the seat of the scornful";

[and] speak against your brother ;
even to pass sentence upon him, to put him to death for professing faith in Christ, (Matthew 10:21) ;

you slander your own mother's son ;
the apostles and disciples of Christ, who were their brethren and kinsmen according to the flesh; and even our Lord Jesus Christ himself, who was bone of their bone, and flesh of their flesh.