Where's tech support? Items keep catching fire.

A few questions: 

• Do the family live in a rental unit? 
• Do you have rental events turned on?

If yes, then you're likely experiencing a failed cursed book event. If so this will keep happening and there is no bug fix. I nearly gave up before I figured it out. 

Solution (though this might take a few tries): 

• Take the family its happening to a communal lot, a park, natural park or something like it. 
• Stay there for at least 12 hours (can be up to 24 hours). 
• Return and see if the problem has gone away. 

For me this took several attempts, and I had to use MCCC to first add the curse (since it was invisible and therefore didn't disappear on its own like its supposed to) to all family members and then let it wear off again. But it did work, and since I did it I have not had any more random fires. 

Once you've removed it, its a good idea to turn rental events off so it doesn't happen again.