We have a Harman P61-2 pellet stove and really like it. The stove is fully automatic, you set the temperature you desire and the stove will cycle on and off by it's self. Pellet usage depends on the ambient temperature, our winters can be mild or quite severe, so I can not give a standard on pellet usage, but never use over four tons. I buy my pellets by the ton through the summer and by the time winter is here my winter heating bill is already paid for.
The pellets are not as labor intensive as wood nor as dirty, but do require a little maintenance. You have to scrape the carbon deposits out of the burn chamber periodically and in the burn chamber there is a removable, thick, cast iron, butterfly shaped, piece that protects the sensors for the thermostat. You must remove this and use a shop vac to remove the ash that accumulates there or the ash acts like an insulator and the stove will not cycle on and off properly. This is usually only done once a year and is not hard, just a little dirty. Ashes must be cleaned out like a wood stove but with far less frequency and amounts. It is not a set and forget stove like propane but it is far better than a wood stove. With a little help with the pellets I think your sister would get along fine with the pellet stove.