This happened to my MIL a few years back. She could have died! Luckily a neighbor came home late and smelled the gas. It's a very dangerous situation because even ringing the doorbell or calling the intercom could set off an explosion. MIL said she was sleeping and woke up with a very bad headache that just got worse and worse, so she just stayed in bed and PRAYED, thinking it was "the end".
We got her a stove that has some kind of device that senses whether the gas is burning or not, and in theory will switch itself off if the flame blows out. She HATES it because it is hard to light (many "snicks" of the piezo things to get the burners going).
Don't know if that kind of stove is avail. in the US. More expensive than the gas sensors, no doubt, but cheap compared to an explosion or conflagration. The electric stove is a good idea, too, but there you risks burns because it's not always obvious the burners are on. With either one something unattended could burn, so make sure no flammable surfaces are around the stove in any case.
Another idea just struck me.. is there a way to wire up the sensors to your parents' electric
That way she can't complain. She might not understand or trust that, though, if she's not "all there".. and rip them out anyway. Would she be likely to get at them if they were installed in the ceiling? If it's city gas (methane) then it will rise; propane will sink.
Then, what if the alarm goes off and yr. parents are out? Is she of sound enough mind to turn off the gas, or get outside, call someone? If you want to keep this tenant your family needs to have an emergency plan that she can understand and implement, and you have to make it clear to her and to her brother that her continued tenancy is contingent upon this. I wouldn't want to kick an old lady out, but you have to do what's right to protect both her and your parents. Maybe there's a possibility of her moving in with her brother. Assisted living is not cheap, and I don't think you can just "have someone put" into it.
I wonder if there are alarm devices that can register remotely.. to the FD or police, or to someone's phone #.
You could also investigate an automatic door-closer?