All sentences are correct in terms of pronoun use. However, if you're looking for a potential issue regarding consistency or clarity in pronoun references, there are no specific errors in the paragraph as provided. Each sentence effectively communicates its intended meaning without misuse of pronouns.
If an answer must be chosen based on possible interpretation of clarity, it could be argued that Sentence 1 is slightly ambiguous since it could lead readers to think the term "his" might not clearly refer to Robert in the context of the brothers. However, it is contextually clear, and the paragraph as a whole maintains a consistent reference.
To directly answer the question as presented, based on strict scrutiny without additional context or guidelines for "error," none would be the outright choice. Please confirm if you're seeking clarity on pronoun consistency or a specific kind of error.