I think the father is the helper, the mother the murderer, the son the victim, and the daughter the witness. Here is my reasoning. I am repeating the 6 clues for reference.
1) The witness and the person who helped the murderer were not of the same sex.
2) The oldest person and the witness were not of the same sex.
3) The youngest person and the victim were not of the same sex.
4) The one who helped the murderer was older than the victim.
5) The father was the oldest member of the family.
6) The murderer was not the youngest member of the family.
Since the father is the oldest, he is not the witness (2). And the son can not be the witness (2). Assume the son is the youngest. He can not be the murderer (6), and can not be the victim (3) or the helper (4). Therefore he is not the youngest.
So the daughter must be the youngest. She can not be the victim (3), or the helper (4), or the murderer (6), so she must be the witness, consistent with (2).
So far we know the father was not the witness, and not the victim (4). He could be the murderer or the helper.
The son was not the witness, but he could be the victim (3) or the helper (1), or the murderer (6).
The mother is not the witness, and is not the victim (3), but could be the helper or the murderer.
But if the father is not the victim and the mother is not the victim, the son must be the victim. And if the son is the victim, the father must be the helper (1) .
Therefore the mother must be the murderer.
Perhaps the son missed his curfew one to many times.