Answer:
This is a simple sentence.
Step-by-step explanation:
The given sentence contains only one independent clause, which is "Megan, my second cousin, and Peter, her brother, walked six miles." This clause has one subject ("Megan, my second cousin, and Peter, her brother") and one predicate ("walked six miles"). It expresses a complete thought and does not contain any dependent clauses. Therefore, this sentence is a simple sentence.