Is it permissible for my sister to unveil in front of the young man's father and paternal uncle, knowing that the young man's father is alive and that the young man's mother married his paternal uncle (her ex-husband's brother) after her divorce from his father?

The questioner's sister is considered a mahram to her husband's father, because he is a mahram through marriage affinity. Thus, it is permissible for him to see what a man may see from his mahrams. As for the husband's uncle, who is the husband of the husband's mother, he is not a mahram to her, and it is not permissible for her to uncover her body in front of him because he is a stranger to her.