ShortRiddles #7 - A murder with in family

There was a family of 4, a married couple, and their daughter and son.

One evening there was a murder in their house and one of these four people murdered one of the other.

One if of the family member witnessed the crime and the other one helped the murderer.

Below were the things we know for sure:

1. The witness and the one 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 was older than the victim.

5. The father was the old member of the family.

6. The murder was not the youngest member of the family.

Then can you tell who is the murder?

Post your comments below.