Another thing to consider which can help us understand this issue is the following:
Incest in the direct line (parents/children) is intrinsically evil.
Incest in the collateral line is NOT intrinsically evil. The offspring of Adam and Eve had to have relations between close brothers and sisters in order to procreate, it was a necessity at the time. However, any type of incest today, whether direct or collateral, is inadmissible due to the ample population in the world today.
Similarly, but in a different way, the Pope considers inadmissible the use of the death penalty in current situations due to the current growth of the faithful in understanding human dignity and the advances on security measures in the world. But the death penalty as such is not intrinsically evil as in times past was permitted.
John 3:27; John 15:5; Matthew 19:26