Hi tonnilerche.
You need to start practising with dildos.
Start with relatively thin ones. I got off to a start using a 12" Doc Johnson double dong.
Try working with this dildo until you are comfortable with pushing it all the way in and letting it out again. The advantage of a double dong is that can be easily pushed out again.
Once you can do that, look for a dildo that is slightly thicker. Repeat the exercise until you can push this larger dong/dildo at least 12 inches inside.
Move up again. repeat.
By now you should be fairly used to having something going deeper inside.
Take it to the next level by buying a small "exxtreme goose".
After you've managed the small goose, try the medium exxtreme goose.
if you can manage the medium goose you will almost certainly be able to manage your wife's hand.
You should also make sure that the lube is going inside.
Use a "lube shooter" or a large syringe (60 ml) to squirt lube into your rectum.
Use a good lube. J-lube and silicone oil are the two best I know for fisting.
Owen also suggested adding jojoba oil to your lube. I've been experimenting with this and it does indeed seem to have a relaxing effect on the supporting muscular tissue.
I wish you luck and more importantly enjoy the ride!