Well, I definitely think there's more to this story than the original post conveys. She says that there was an altercation between the stepfather and the grandparents. If there were problems, then why was the step involved at all? Also, is there an investigation into the claims of abuse against the stepfather, or were they dismissed?
While I'm no expert on gpv, I'm not convinced that her case is a slam-dunk either.
It depends on the gp. I can tell you that the gps in my case made the colossal mistake of searching out other family members to attempt to corroborate a fabricated story that my husband was abusing my children. They mistake was that they approached my children's stepmother who happens to be my best friend. She laughed in their faces and informed them that she WOULD be telling me about that. They drug my husband into the situation as much as they possibly could.
As far as the slam dunk....I agree. OP is still "new" into the gpv situation and we ALL spew that when we are confronted with the situation originally. There is more to the story that the OP is NOT telling us.