What would you like to have happen? In your description, I do not see you alleging that you are a victim... Just your sister.

This might explain why the police are not listening to you and your brother's reports as much as your sister (who, it seems is the victim).

The sex of the possessor of child porn is irrelevant. But, the subject matter of naked pictures of one's own children might introduce a gray area, and hence the different interpretations. I REALLY don't want to know more about it, and you explaining more won't help the situation. It's also hard to convict a person of a crime, which is described as "WAS in possession of".

It certainly sounds like you have a lot of unresolved issues, that really should be addressed. A good place to start would be therapy. You may be chasing a resolution which will never bring you closure, and help you heal. Also, your therapist would almost certainly be a mandated reporter. If the therapist suspects that there might be a danger of ongoing abuse, they would be required to report it to authorities. And, a report from a therapist would be taken more seriously.