I have absolutely no idea, that's something only you can answer!!
The TiVo boxes are quite old now, and whilst being perfectly serviceable, if you have VM broadband I'd recommend trying to swap them for V6 that run the same software but on much newer, more capable hardware.
That said, only you know your mother-in-law's circumstances, what channels she watches and how she uses the boxes. As a starter for six, check which channels she regularly watches and/or records, and compare it with the package guide at the bottom of this page
If she mainly watches the "freeview" type channels then she needs to decide how much the recording capability is used, compared against a simple Freeview recorder versus whether VM will offer an acceptable package on the base Mixit bundle. If she watches any of the pay-channels, then it's probably a shoot-out between VM & Sky.
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