Under declaring one's earnings

My father is a business man. He suppresses his financial state to reduce income tax claims as all other businessman like him do. Many large traders often hire a lawyer who could suppress their financial state intelligently to minimize the income tax payable on their income. As we are part of this country and are required to abide to all the laws as per both religious and ethical requirements, are we committing a sin by not paying the tax on our true financial state? Our government holds right on our income and it is our obligation to pay tax. However, this is a common practice in my country.