I am a retired CPA. My mother in law has requested that I prepare her income tax returns starting with the 2024 tax year. Can I do this using TurboTax?

You can't use Turbotax as a paid preparer.  

You can use it as a "family helper" but your MIL will have to sign her own return.  Of course, if you e-file, her "signature" is her past year AGI, and there's no way for the IRS to know whether or not she really typed it in, unless she complains later.  When I did my ex-wife's return for her in April this year, I got on a zoom call with her and screen-shared so she saw everything I had done and approved as I was filing.

The technical license terms are that you can install the program on up to 5 computers that you own, and you can prepare as many returns as you want, as long as you don't charge for your services.  The first 5 returns can be e-filed, if you have more than that, they must be mailed.  

If you want to prepare more than one return, it will be more cost-effective to buy Turbotax Deluxe Federal+one free state version as a CD or download to install on your own Mac or PC, rather than using Turbotax online, unless she has a simple return and qualifies for free online filing.   With the Deluxe Fed+ 1 state version, you can file an unlimited number of federal returns (first 5 can be e-filed). You get one state module for free, each additional state module costs about $45.  And each state e-filed return costs $20 or $25 (you can print and mail for free).  Even with those costs, it's still more cost effecting than using the online program because each online account can only file one return and each has separate fees.