[Grammar] Definite/Indefinite Articles

[rainous]

Brian talked with a brother of one of the victims, the sister and daughter of the other two, and with the father of one of the killers.

In the sentence above, can anyone please explain to me why, in the first instance, the indefinite article "a" was used while, in the second instance, the definite article "the" was used?

How do you decide which one to use?

Or is it just a random thing that makes sense either way?

Thanks

 

[5jj]

The implication is that the first victim had more than one brother and the second had only one sister and one daughter