Final answer:
The first doubt about the king's authenticity as Peter Wilks's brother arose from his inability to answer questions about the Wilks family.
Step-by-step explanation:
The first thing that cast doubt on the authenticity of the king being Peter Wilks's English brother was his lack of knowledge about the Wilks family. It was expected that as Peter's brother, he would be able to answer questions about his supposed family members and their lives. The king's inability to do so raised suspicions among the townspeople, suggesting that he might not be who he claimed to be.