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.

From (3), the youngest person and the victim were not of the same sex. So mother can't be victim. 4th possibility eliminated because of (4) as well since as per it helper is older than victim.

From (1), the witness has different sex than helper. So only 1st possibility can be reality.

To conclude,

Mother is Murderer,

Son is Victim,

Father is Helper

and

Daughter is Witness.