We can set you down the right path.
First off, it's not a "powerbook" -- it's a "MacBook Pro".
Take a look at it and tell us -- does it have a DVD drive slot on the right side?
If it does, it's a "Unibody" MBP (drive and RAM are upgradeable).
If it does NOT, it's a "retina" MBP.
In either case, it's possible to "wipe it clean" and start fresh.
And STILL "bring all your data over" from the 2014 Mini.
Hmmm... tell us about your Mini.
How much RAM?
What kind of drive is inside?
Here's how you "reset" the MBP:
FIRST THING:
Get your wife to boot it up.
Now, SIGN OUT of iTunes and remove it from her iTunes.
Next, she needs to sign into iCloud and remove it from her iCloud, as well.
Otherwise, you could put a new OS onto it, and it will still "think" that it's HER MBP, and not "yours".
Once that's done, you can boot to something called "Internet recovery" and wipe the drive clean, and then install a new OS.
With the power off, press the power on button and IMMEDIATELY hold down:
Command-OPTION-R
You will be asked for your wifi password -- enter it.
Then, be patient "as the globe spins" and loads the internet utilities.
When you get to the internet utilities, open disk utility.
VERY IMPORTANT step: Go to the "view" menu and choose "show all devices".
Now, look at "the list on the left".
The topmost item should be the physical drive inside.
We need to erase it.
MAKE SURE YOUR WIFE DOESN'T NEED ANYTHING THAT'S ON THE DRIVE.
We are going to ERASE it -- it will be gone forever!
Click the "erase" button. Erase the drive to "APFS, GUID partition format".
Erasing takes only a few seconds.
Quit disk utility.
Open the OS installer and start "clicking through".
The Mac will restart one or more times, and the display will "go dark" for a minute or more, with no other indication of activity. BE PATIENT.
When done, you should see the initial startup screen, just as if it were new -- "choose your language".
Start clicking through.
IMPORTANT:
Do you have a backup of the 2014 Mini?
Do you want to bring "your old account" over?
You should have a backup to connect at this point.
If you don't have one, you could use an external hard drive to MAKE one.
I would recommend using "SuperDuper", which is free to download and use for this purpose.
ANOTHER WAY:
You could start "completely clean", with a clean account, but then you'll have to migrate apps and data "by hand". This is more work, but it can be done.
I suggest you print out this reply so you can have it nearby as you work.