If you are a distressed party, you may opt for investigation as a stakeholder by demanding for it in the company general meeting or by writing to the partnership firm. If you are not a distressed party, you may not be able to proceed against your brother in laws demanding for investigation. You may, however, as a whistleblower, write to the Investigation Department of the Income Tax Department with whatever evidence you have in possession about the tax evasion.
If there is no issue relating to tax evasion, you may try to sort out the issues amicably with your brother-in-laws, as the blood is always thicker than water.
You may engage the services of a CA for investigation at an appropriate stage.
