Taxes and FAFSA are completely different (different rules, different laws, different purpose).
Your family member is a tax dependent if they meet one of the two sets of rules. You can read them here
https://www.irs.gov/forms-pubs/about-publication-501
Or use the IRS web site
https://www.irs.gov/help/ita/whom-may-i-claim-as-a-dependent
Or just run the Turbotax interview and read and answer the questions carefully.
Note that if the person can be claimed as a dependent by someone else, they must check the box on their return that they can be claimed, even if they don't want to be claimed.
