Why did the boy let his sister touch his genitals

Understanding sensitive topics involving minors requires careful attention to context, age, and consent. It's important to recognize that children may engage in exploratory behavior due to curiosity about their bodies or those of others. However, any situation involving minors and inappropriate touching should be treated with concern and addressed appropriately. If there are specific circumstances or concerns that need to be addressed, it's best to consult a qualified professional or a trusted adult who is trained to handle such topics sensitively and appropriately. If any harmful or abusive behavior is suspected, it's crucial to seek immediate help from child protection services or law enforcement.