You can do with your Kobo e-reader what your wife does with her Kindle Fire. Set up your Kobo account first by going on your PC (faster that way, for typing all your info) to kobobooks.com. You can sign up using your email address and set up all your personal info (password, address, etc), add a credit card/gift card, etc. Then you can buy books while logged into your account on your PC or directly from your Kobo ereader (via WiFi) by going to the 'store' on your ereader. Your ereader stays 'logged in' to your account and you don't have to type in your password each time you buy a book to add to your library.
You can also side-load books via your PC (which is essentially connecting your ereader with a usb cable to your PC and copying epub books from PC to your ereader). The only time you need Calibre is when you're converting books from one format to the other. So, for example, if your wife bought some Kindle books, they would be in .mobi format, and you could use Calibre to strip the DRM and convert to .epub and then side-load. I don't bother with this! I just have a Kindle app on my tablet and phone and read Kindle books from there. I am too busy to d*ck around with stripping and converting (ha! That sounds dirrrty!).
You can also sideload other books/library books using Adobe Digital Editions. Again, I just prefer to download those ones directly to my phone/tablet and read via Aldiko or Overdrive.
Your books will always be available in your Kobo library account tied to your email, and you can read your books from any device (either from a Kobo device, or from your phone/tablet with the Kobo app). For example, I may read a Kobo book on my phone while waiting for an appointment (using the Kobo app). When I am done reading, my 4G signal has me 'connected' to my account and will update bookmarks when I am done reading. So - when I go home and continue with my book one of my Kobo devices, I can pick up where I left off, after my library is done updating.
Enjoy your new Kobo! The mini-could be cool - but if I were you, I'd buy the glo! That's the best of all worlds (still compact, can read in low/no light, no glare in the sun, easy on the eyes). I am going to ask Santa for mine!