There are a lot of valid points here. I just want to say that it is not ALWAYS a bad idea, but there needs to be value brought to the table from all parties.
Our business was started by myself and my brother and we have continued to grow it together, successfully. Our 3rd brother wanted to join in but didn't know a thing about real estate and as a result had nothing to bring to the table.
Our recommendation to him was to learn, learn, learn and come back with some value to bring to the company. He now does our marketing and SEO and is working his way into an even partnership.
My advice would be to hold them accountable and force them to grow before seeing the fruits of the labor. Make them learn a skill, learn real estate, do something that provides long term value to the business. The worst thing for a business is if one person feels they are doing the work of the other partner(s).
It may not be ideal or work for many, but it is possible when done the right way.