What is the ruling regarding someone who thought their brother was listening to the Qur’an, but he was actually listening to shilat, and so he said to himself: "Here comes this entertainer," then he feared that sin might result from his words?

It appears that the questioner is suffering from waswas (obsessive whispering/thoughts), so he should cast off these thoughts and not succumb to them. Waswas is a severe illness, and succumbing to it leads one into confusion, pathological doubt, distress, and religious constraint. Just as it is obligatory to beware of ridiculing the Quran or religion, one should also not be someone who is mawsoos (afflicted by waswas), accusing himself or others of disbelief every time something happens.