Wild guess: Insurance against necromancy, cancelled when one or both of them died and their remains dealt with in a way that made that impossible (e.g. cremation)?
Nope. The focus on supernatural beliefs is a good idea, though.
Does it have anything to do with reincarnation?
No, not reincarnation.
Is death involved in any way?
Were they insuring themselves against being turned into zombies?
Nope.
No.
Hint #2: You have confirmed it involved supernatural beliefs. I will now ask
Is that belief religious in nature?
Definitely.
Does it involve the Rapture?
Insuring against being left behind? Being taken?
No to all of this.
Does it involve a belief held by at least one of what we would think of as the major world religions?
If so, would it be considered a “core belief”?
Would it be considered heresy?
Yes in the general sense.
No in the sense that these sisters belief was not widespread. But it involved something a major world religion has in it.
Not what the sisters believed, but the overall concept is a core belief
I suppose the sisters belief is heresy, certainly not a view held by those with the same faith.
Does it involve God?
Jesus?
The Devil?
It involves Jesus.
Does it involve the Second Coming?
Not in the traditional sense, no. 
Were they insuring themselves against immaculate conception?
No, but I’m going to give it to you because I think you mean supernaturally becoming pregnant with Jesus Christ(look up immaculate conception).
Anyway, consider this solved. Three sisters took out a policy to cover the costs of having to raise the Christ-child in case one of them conceived Jesus Christ. The policy was approved, but was specifically cancelled due to pressure from the Catholic Church.
Okay, how about another video game puzzle? Again, don’t shout it out if you already know it.
A point-and-click adventure game in the '90s had the player solve one of its puzzles by using a specific tool to shut off a water pump. Sounds simple, right? Yet players outside of the US found the puzzle nigh impossible. Why?
Was the tool not used for the same purpose outside the US as in the United States?
The tool is used for the same things regardless of country.