Book the third, Chapter 10
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They are rich aristocracy who had stolen a woman and raped her. They have her confined to a room. They think she is going to die so they call Dr. Manette.
They are no other than Charles Darnay's own father and uncle. The elder brother is supposed to be Charles's father and the younger one, the Marquis, who was shown as very brutal and murdered with a knife in his chest. Since Charles inherited their blood, he is tried and sentenced to death by the people of the Republic.