Halakha Ulangan 27:16: Chafetz Chaim, Kitzur Shulchan Arukh & Sefer HaChinukh

(And if, G–d forbid, the lashon hara were against his father and mother, he also transgresses a fourth arur (Ibid 16): "Cursed is he who demeans his father and his mother," which we have already explained above (Positive Commandment 14) in the Mekor Hachaim and in the Be'er Mayim Chayim.)

(10) And if he [the one spoken against] were his older brother, or his mother's husband or his father's wife, he also transgresses the positive commandment of "honoring," their having been included [in this mitzvah by the addition of "ve'eth", [(Shemoth 20:12): "Honor your father and [ve'eth] your mother," as explained in Kethuvoth 103a)]. How much more so — if, G–d forbid, [he speaks lashon hara] against his father or mother themselves, where he certainly transgresses the positive commandment of honoring father and mother — [is such lashon hara especially egregious]! Aside from all this, he also transgresses (Devarim 27:16): "Cursed is he who demeans his father and his mother" — may Heaven protect us!

Whoever disgraces his father or mother even by words or gesture is among those whom the Almighty has cursed. As it said,⁹Deuteronomy 27:16. "Cursed is one who disgraces his father or mother."