You can certainly share your relatives' TurboTax Online accounts if they give you permission. They would have to tell you their user IDs and passwords. TurboTax wouldn't even know that it's not them using the account.
But with your background you might not be happy using TurboTax Online. It lacks many of the features that are in the CD/Download software. The biggest difference is that TurboTax Online does not have forms mode. There is no way to see the actual tax forms except by creating a PDF of the return, and there is no way to make entries directly on the forms. All entries have to be made in the interview. The lack of the forms mode also means that certain other features are not available, such as Data Source and QuickZoom to trace the data flow and see how numbers in the return were calculated. And there is no way to view or print a form that is not required for the tax return.
Also, TurboTax Online shuts down shortly after the October 15 extended filing deadline. When it comes back up in early to mid December, it's only for the next tax year. So there's a period of a month and a half or two months when you do not have access to the online returns at all. After it switches to the next tax year in December, you can only download the tax returns from the previous year (the PDFs or the .tax20yy files). You can't open them to review the entries or manipulate them in any way. You are strongly advised to download both the PDF and the .tax20yy data file after you file each return and save them on your own computer in case there are any problems accessing the online copies.
