If one speaks lashon hara about a kohen and denigrates him through this, one transgresses the positive mitzvah of (Vayikra 21:8), "You shall sanctify him," which commands us to treat kohens with much honor. By speaking lashon hara against him, one is certainly not honoring him.
If one speaks lashon hara about one's oldest brother, or the husband of one's mother, or the wife of one's father, one also transgresses the positive command of, "Honor your father and you mother." From the extra word "et," it is derived that these people must be honored as well.
How much more, heaven forefend, that speaking lashon hara about one's actual father or mother certainly incurs the transgression of the positive mitzvah of honoring one's father and mother. Besides this, one also transgresses (Deuteronomy 27"16), "Cursed be he who denigrates his father or mother."
