If you are a small business, you are required to have an EIN if you pay employees. Otherwise an EIN is optional, but it is useful to have since then you can give your EIN to customers and vendors instead of your personal SSN.
Your EIN is linked to your SSN. You should never let someone else use it for their business. That would be the same as letting them using your SSN.
Your family member should use their own SSN for their business, or they can apply for their own EIN at the IRS web site.
