My mother was diagnosed with dementia and has appointed me as her DPOA. She requested that I do her taxes. Can I do it through my Turbo Tax account or do I create a new one for her?

The TurboTax online editions are only for completing one tax return per account and User ID.  You will need to create a new account with a new User ID to start another tax return using the online editions.  You can have up to 5 User ID's associated with one email address - https://turbotax.intuit.com/personal-taxes/online/