Hathcock told police that the intervention went "very wrong" and the siblings got into a verbal argument which escalated into a physical one, court documents said.
"It is unclear who [threw] the first punch but Hathcock kept repeating that she is the one who is at fault and she used her long fingernails to rip open [Rosenbergs'] testicle," Officer John Nicoline wrote in the arrest affidavit.
Police noted that Hathcock was "very calm" at the scene - her brother, not so much.
Rosenberger had fled and was found hiding in a parking lot behind a nearby Comcast building, according to police. He was described by police as being "upset," "in a lot of pain" and "bleeding from his crotch."
Police had to help Rosenberger into an ambulance and he was taken to Phoenixville Hospital, where he received seven stitches to his right testicle, according to police.
Hathcock was charged with aggravated assault and harassment. Police also charged Rosenberger with a single count of harassment, according to court documents.
Rosenberger and Hathcock did not return calls for comment. It's unclear how Hathcock was able to allegedly grab hold of and slice open her own brother's testicle.
Contact Stephanie Farr at email@example.com or 215-854-4225. Follow her on Twitter @FarFarrAway.