steve: Complicating factor:
In Scenarios 1 an 2 the sisters tell you in no uncertain terms NOT to phone anyone. They just want to go home. Especially for victims of sexual assault, they have a reasonable fear that the aftermath will be taken out of their control and they'll be re-traumatized. So they go from being controlled by a perpetrator to being controlled by others (ambulance staff, police, nursing staff).
Complicating factor: how young is young? Are the "young sisters" minors? (well...not the dead one...dead is dead regardless of how old you are)
If the sister was a minor, there is no option to let her decide to report the matter. When a crime is committed against a minor child (under 18 in most countries), it must be reported. A minor does not have an option to say "don't call the police".