Jesus didn’t have any brothers nor sisters, so I don’t understand how we could trace anyone back to him?
The question is not about genetics, but about the validity of a priest’s sacramental Holy Orders.
The way it works is this: a priest’s Holy Orders are valid only if he was ordained by a valid bishop. And a bishop is valid only if he was consecrated by another valid bishop. While on earth, Jesus made the twelve Apostles bishops. So the original poster is asking, for any priest today, can we prove (1) that he was ordained by a bishop?, and (2), in order to show that that bishop was valid, can we prove an unbroken series of episcopal consecrations back to one of the original Apostles (this is what is meant by the phrase “apostolic succession”)?
In other words, can we prove that one of the Apostles consecrated some bishop, who consecrated some bishop, who consecrated some bishop, … etc. etc. for twenty centuries …, who consecrated the bishop that ordained this particular priest? Unfortunately, we do not have the records to allow us to do this. However, we believe it did happen – that every bishop today was consecrated by somebody, who was consecrated by somebody, who … etc. etc., who was consecrated by one of the Apostles.
Why do we believe it without documentary proof? For two main reasons: (1) the evidence we do have shows that people have always taken the issue of apostolic succession very seriously, so there is no reason to think that they just gave up caring about it for a few of the centuries from which we don’t have records; and (2) because we believe that the Church is divinely instituted and guided by the Holy Spirit (and “the gates of Hell shall not prevail against it”), we believe that God would not have allowed all our bishops and priests to have fallen into invalidity at some point in the distant past.