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Solve It Genius

One evening there was a murder in home of married couple,their son and daughter
One of these four people murdered one of others.one of the members of the family witnessed the crime.

The other one helped the murderer.

These are the things we know for sure

1.The witness and one who helped the murderer were not of the same sex

2.the oldest person and witness were not of the same sex

1. The youngest person and the victim person were not of the same sex

2. The one who helped the murderer was older than the victim

3. The father was the oldest member of the family

4. The murderer was not the youngest member of the family

Who was the murderer
Who was the helper
Who was the witness
Who was the victim

Think before answering