I'm thinking of starting up the same type of business as my brother-in-law after working for him for 3 years
Why would you take his idea?
I think I can do it better than him.
You should not take ideas from your brother. It is unethical to do so and it is a betrayal of trust.
It's my idea. I worked at his store, I know his work and now it's time to make some real money.
If you take his idea you could hurt your family dynamic and create drama. Have you considered what other sacrifices like time and money it took to even set up the business? Do you have someone you can get funding from?