Maybe your mothers Glock is a Glock 23? The 23 is the compact but it still retains the 3 finger groves (where as a sub compact has only 2 finger groves) although they are scrunched closer together, while the 22 has the 3 finger groves, but more room for larger fingers as it's a longer "full sized" grip. If your hands are so big that your ring finger is only half on the grip on a full sized Glock 22, then I think you'll have a hard time finding ANY handgun that would fit.
If your Mom's Glock is indeed a 23 and not a 22, then I'd expect you could probably get away with a Glock 22. If you are correct and your Mom's Glock is a 22, then You're a Giant and I wish you luck! No full sized handgun, no matter how thick of a grip (such as an original Glock 21) will make up for the extra length you need from the grip to get the rest of your ring finger and pinky on it.
If that really is the case, maybe something with a full sized grip plus grip extensions/magazine extensions on your magazines would help out.