Insuring a sub-contractor?

You would not need to worry about workers comp if your step-brother is truly a subcontractor. He should get a general liability policy with limits at least as high as your own. If you add him to your policy he would generally have to be considered an employee. You may also be required to be listed on his policy per your insurance company. The best thing to do is make sure that your insurance agent is reputable and knows what is going on. I don't recommend getting a contractors policy online.