The other one helped the murderer.
These are the things we know for sure:
The witness and the one who helped the murderer were not of the same sex.
The oldest person and the witness were not of the same sex.
The youngest person and the victim were not of the same sex.
The one who helped the murderer was older than the victim.
The father was the oldest member of the family.
The murderer was not the youngest member of the family.
Who was the murderer?