In 25:11 the passuk tells us “Pinchas, son of Elazar, son of Aharon Ha’Kohen”. Rashi tells us the reason the Torah felt it important o state the lineage of Pinchas back to Aharon (the grandfather) instead of just to Elazar (the father) was because people were talking about how hypocritical Pinchas was. After all, they said, Pinchas’ grandfather (Yisro) himself was a priest o Avoda Zaras who fattened the animals for sacrificing to all sorts of deviant gods. Who is Pinchas to preach to us??!! Therefore the passuk relates Pinchas to Aharon.
So what? What does relating him to Aharons side of the family do for anyone? Did people stop talking after that? It must have gone something like this: People were talking about Pinchas and then suddenly they read he was Aharons grandson so they stopped??!! I do not think so. They knew he was Aharons grandson to start with, so what was the purpose of writing this?
I think the passuk was not writing this fact in order to stop the rumors and lashon hara from spreading and to stop people from besmirching the integrity of Pinchas. People talk and nobody likes to be told they are acting inappropriately, so they spoke badly of Pinchas. The passuk is coming to tell future generations – us, that we should know that the intentions of Pinchas were honest and done with integrity.