Start with your father-in-law and make sure he introduces you to all the people he deals with so they understand you will be taking over. The transition phase. This means being CCed on emails and such. If he does not use email then making sure you are on conference calls with whoever he deals with.

Before you go and try to change anything make sure you have a good handle of the business. Understand how the business is currently run.

Once you understand how the business is run. Start talking to employees about what can be improved. What are the pain points?

Not knowing what your business does makes it a bit difficult to provide more suggestions. If you want to chat feel free to reach out.