What is the ruling on popular building (without a permit) on land owned by the questioner? And is he sinful for paying an amount to a municipality employee in exchange for not reporting the renovation of his brother's house?

Violating laws regulating construction and urban planning, which serve the public interest or prevent harm, is impermissible. Paying a bribe to achieve this is an additional sin. Therefore, you must seek Allah’s forgiveness and not repeat such an act.