There are four details about Lashon HaRa that, according to the Chafetz Chaim, we learn from the episode of Miriam’s words against Moshe.
First, she had saved Moshe’s life when he was a baby, so he owed his life to her. Nevertheless, she had no right to speak critically of him. Second, she said this only to her brother, who was also Moshe’s brother. Clearly Aharon was not going to spread this around; it was to stay between them. Despite the local nature of this infraction it was still inexcusable. Also, the Torah tells us at this point that Moshe was the most humble of men. Miriam was still wrong to criticize him. Finally, the Chafetz Chaim notes that Miriam didn't say anything completely negative, but only that even though Moshe was great, he was perhaps not on a high enough level of prophesy to act as he did. This was still Lashon HaRa.
I'd like to add a fifth point; if you are very close to someone, that does not give you free license to speak against him or her. Miriam was punished even though the Loshon HaRa she spoke was about her own beloved brother.
These lessons about Lashon Hara remain relevant. Often, we feel like we have a right to speak about others because they are under our charge. This applies to all of us at one time or another, whether as teachers, parents, coaches, siblings or friends. While others should appreciate the good we do for them, goodness does not buy us right to criticize, even if we saved their lives.
If you tell someone who won’t tell anyone else, you still spoke Loshon HaRa. There is an internal damage, which occurs when you say Loshon HaRa, regardless of whether the external harm is great or small. While it is true that there is a mitigated heter to speak to someone in confidence for therapeutic reasons, it behooves us to not jump to rely on that leniency in a careless, common way. You never know what is inside a person, and it is possible that self-effacing people who seem to mind least are hurt most of all by Loshon HaRa.
It is possible to rationalize Loshon HaRa by reframing it as constructive criticism. If we genuinely have to criticize someone we should ideally say it kindly, gently and face to face. Criticizing someone behind their back is Lashon HaRa.
We often think that Loshon Hara, or embarrassing people, doesn't apply to those with whom we are closest. The truth is that those we are closest to are often the most likely to be hurt by what we say. If we speak sharply about acquaintances (which we generally don’t do), it doesn’t sting strongly. But our brother, sister, friend cares what we say. It is the people we are closest to that we need to be most careful not to hurt.
I have been telling and writing this Dvar Torah for twenty five years plus, since I first read it in a borrowed copy of Love Your Neighbor in the summer of '80. And yet. It is so hard to internalize. So hard. When I posted it a few years ago on my personal blog someone commented critically and hit a raw nerve...
May we be blessed to be sensitive in how we speak of others and to learn from this parsha how careful we truly have to be.