After Moshe leads Israel in songs of praise after the splitting of the sea (az yashir), Miriam leads the women in similar song.
The passuk in 15:20 says, "ותקח מרים הנביאה אחות אהרון את התף" - And Miriam the prophetess, sister of Aharon, took the drums in her hands, etc..
Why does the Torah relate to us that she is the sister of Aharon? We know who she is.
Rashi explains that she said her prophecy before Moshe was born so it relates her to Aharon, along with a second explanation that Aharon risked his life for her when she had tzoraas, so it relates her to him.
That is very nice, but does not explain why it needs to be said at all. That explains why she is considered Aharon's sister and no mention of Moshe.
But why say anything? We know who she is!
The Ramban says an answer that we already have seen Moshe leading Israel in songs of praise, now we see Miriam doing the same. The only one we do not see is Aharon. So the Torah relates Miriam to Aharon so he would not feel left out.
I find it funny that the Torah would say that just so Aharon does not feel left out. Is this a personal family diary that the author needs to be concerned with hurting one members feeligns over another?
I asked this question to someone and he suggested that it is teaching us a lesson. He said that if the Torah showed concern and compassion for someone who might feel left out when his brother and sister got all the attention, how much more so do we need to show compassion to others and be concerned for their feelings.