The Torah obligates us to vocally recall the punishment Miriam received for speaking loshon hora about Moshe Rabainu. There were numerous factors that might have mitigated the gravity of Miriam's sin and thus her punishment.
(1) She spoke about her brother whom she loved dearly.
(2) She risked her life to save Moshe when he was an infant.
(3) She raised him in his childhood.
(4) She did not say anything actually derogatory about Moshe: she merely minimized the extent of his greatness.
(5) Since she did not speak in Moshe's presence, he did not suffer embarrassment.
(6) She did not speak against him in public, she privately spoke to Aharon, her brother.
(7) Moshe Rabenu was the paragon of humility and was not affected by what Miriam said about him.
Nevertheless, despite her righteousness Miriam was punished with leprosy. Anyone who speaks loshon hora violates the commandment to recall what HaShem did to Miriam.
