This is a tricky one!! Another puzzle to follow soon 🙂
We know from (3) that the youngest person was not the victim, from (4) that the youngest person was not the helper and from (6) that the youngest person was not the killer. The youngest person can only have been the witness therefore. If we make up a chart there are now three possible combinations:
Oldest person (father) H H M
Next to oldest (mother) V M H
Next to youngest (son) M V V
Youngest (daughter) W W W
(H = Helper ; V = Victim ; M = Murderer ; W = Witness)
We can work out from (5) that the father was the oldest, from (2) that the youngest person must have been the daughter. Therefore the next to the youngest must have been the son and the next to the oldest, the mother.