I'm going to have my 13 year old brother sell merchandise for me.
What are you going to have him sell?
I'm going to have him sell drugs for me.
This is wrong in many ways. Selling drugs is already illegal and can get you and your brother in trouble with the law, but your brother is also 13. Even if it was legal merchandise, you can't have someone that young working for you.
I know, but we're both going to make a lot of money with this.
The money can definitely be tempting, but you should consider the risks involved in selling drugs.  You could both be arrested, even worse, your brother could be killed by other criminals.  Try to find a business venture that you can both work on together, safely and legally.