I remember this show when it was on the old CBN Cable channel back in the mid 80s. Good show.

However I remember reading in a book years ago saying how nasty and rude Joan Davis was.
How Jim Backus really hated her and that Joan Davis' daughter who played her sister (Beverly Wills ) both her , her sons, and Joan Davis' mother all burned to death in a freak fire not long after Joan Davis died, who died fairly young too.

any of this stuff true?

It wasn't really all that soon after Joan Davis' died of a heartattack, but her daughter, her sons, and Joan's mother did die in a fire in 1963. Beverly was only 29 at the time of her death.

On Jim Backus, I'm not sure.

Ive seen somewhere how much hatred Jim had for Joan.