Roles of Family Members
What was the case?
From (6), we know the youngest one wasn't the murderer. Clue (3) suggests youngest wasn't the victim & (4) hints that youngest wasn't helper too. Hence, youngest must be witness.
So possibilities of others' role are listed in table below.
From (2), the witness had opposite sex than the oldest & from (5), we know father is the oldest. So witness must be daughter.
From (4), murderer was older than victim & since father is oldest he can't be victim. So last 2 possibilities are eliminated.