You absolutely do not want to do this. You are exposing yourself to liability and fraud claims by your ex. You should sit down with an experienced business attorney to clarify the risks in entering into this type of arrangement.
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With your additional facts added to your prior posting, I still think it is very risky and a poor idea.
You are taking the word of someone circumventing the law that he will pay insurance, taxes, hire licensed drivers and that doesn't even address your family law issues.
The above is general legal and business analysis. It is not "legal advice" but analysis, and different lawyers may analyse this matter differently, especially if there are additional facts not reflected in the question. I am not your attorney until retained by a written retainer agreement signed by both of us. I am only licensed in California. See also avvo.com terms and conditions item 9, incorporated as if it was reprinted here.
This has "see you in court" written all over it, but I suspect you know that already which is why you are posting the question.
It would appear that your family member would suffer the greatest exposure. Should your ex-wife discover the business interest in your name she can add that to the list of assets to go after. She may not be entitled to any managerial or voting rights in the business but she sure can go after your financial interest in the company and this presumably would really be your relatives interest.
I suggest you consult a lawyer in private to explore your options in more detail.
Most of us here, including myself, offer a free phone consult.
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