Jpoe,
There is more to this than just a pistol/weapon for defense.
It has to start with a mindset.
Neither of my parents carry a firearm but are indifferent towards them
They have to get out of their indifference and into a survival mindset.
A firearm is good. But won’t do them any good if an intruder is able to sneak up on them in the night.
I have early warning devices. A dog, Security Alarm, motion activated lighting. My house was built in the 1800’s so the wood floors are impossible to walk on w/out making noise lol.
Locks on all doors to sleeping quarters, etc.
Most importantly? A plan. A mindset. I run through scenarios of “what if, when this, happens” type stuff.
Your parents have to do this or they may not be victorious in a confrontation inside their own home.
To me, Security is a lifestyle. It’s not something every now & then, or a hobby.
IMO, this is the first thing they have to understand regarding defense.
I’m of the opinion small guns are for carry only. If they wont carry it, I think the sd9 would be a great option. Basically, you would want the biggest they can handle well as its easiest to shoot.
To be honest, I’d suggest an ak with a light. That would beat the crap out of a handgun and they are much more likely to be able to hit where needed under stress, but that bumps you up to 500 for the gun.
Small guns have a small niche, IMO.
His step father is 75 yrs old. Personally, I’m not sure he is up for the Rock’nRoll of an AK 
Maybe a shotgun with mounted light? Low brass ammo?
Dad would have to be assessed first to see what limitations he has and figure that into the purchase.