Hi Lee
That seems a good linked article.
Some random thoughts-- One must practice somewhat to be accurate enough to 'safely' use a handgun, so the firearm ought to be comfortable enough to run some rounds thru for some practicing.
If the arthritis reduces finger strength, then a light trigger pull would be desirable.
On the other hand (and I may be too conservative here)-- I have some pistols with light triggers that I use for target practice, but the ones I carry or would grab the middle of the night, are double-action with stiff action. A self-defense shooting is about as serious an action as a civilian might ever take, and I don't want a trigger action light enough that there is much chance of accidentally firing the weapon under stress. But unless someone is REALLY GOOD, target-practice accuracy suffers with a stiff double-action trigger.
When taking concealed-carry course, the instructor told of a Glock lawsuit case of a fellow who owned (IIRC) a pizzaria, who bought his Glock and never fired it at all. Just loaded it an put it under the cash register for defense.
Later on, defending a robbery, the fellow fired the first round so limp-wristed that the action didn't cycle correctly, and the robber shot him after the store-owner could only get off one round. The customer sued Glock because the gun was 'defective', but Glock won because it was judged that the shooter was the defective part of the system. However, if someone with arthritis doesn't have sufficient wrist strength to hold a semi-auto fairly rigid, then maybe they could have the same problem of the weapon not cycling properly?
Maybe a double-action Glock for the mother-in-law, since the action is simple, but much lighter/smoother than most double-action pistols? Assuming her wrist is strong enough?
The S&W J-Frame revolver grip is the 'easiest to grab and hold tight' grip I've used. A very secure, comfortable grip. Maybe a .38 stainless S&W J-Frame? They are heavy enough that .38 recoil ain't all that bad. The factory trigger is usually pretty stiff, but a good gunsmith could inexpensively do a trigger job to smooth up the action and optionally lighten it up a little.
This S&W 649 has the style of grip I like so much-- Slim & long with the three finger slots (S&W has some other style grips on some other J-Frames)--
http://tinyurl.com/2alhq7
Had trigger jobs done on the S&W 586 and the little 649, and they work pretty smooth double-action. In single-action, the trigger is so light that you really don't want to cock the thang unless it is pointed at something you already definitely decided to shoot-at. But the single-action pull was really light on those two specimens before the trigger job, so the trigger job didn't really make the single-action 'more hair-trigger' than it was from the factory.
Unless mother-in-law wants to carry in her purse, a revolver with a longer barrel ought to be as-good-or-better for home defense. More accurate. But that style of grip is to be desired IMO, to keep a good hold on the thang.
Here is a 5" barrel J-Frame revolver (they also have a 3" barrel, in addition to the snubbies):
http://tinyurl.com/25xv5s
A dedicated .38 might be better than a .357, though .38's shoot fine in the .357's. When first learning, I shot a few hundred .38's in the 649. Thought I was cleaning the gun adequately, but after hundreds of .38 rounds, there was enough 'hard to see' burnt-on residue in that deepest tenth inch of the cylinder, that it took a REALLY dedicated scrubbing before .357's would chamber. So if mother-in-law got a .357 to exclusively shoot .38's, and then you eventually inherit the gun, it would be a serious cleaning before you could use .357 in that gun...
