creative writing - How to determine whether or not a plot twist is needed?

I should start by saying that this question is not this question, How to decide if a plot twist worth doing despite how similar they sound. And, the answer I'm looking for was not at this question either, How/When to include twists when developing plot.

So, here is the situation. The novel that I'm working on is very close to completion; the climax and falling action are done, and it's in its resolution. But, to avoid ending the book too quickly and leaving the readers with questions unanswered, I had thought of a twist to put into the book.

I'll keep the explanation of the twist as simple as possible:

There are 8 main characters trying to escape a very dangerous situation. So dangerous, that one of the characters perishes in the escape. Now the other characters didn't like this character very much, one of those other characters liked her the least. I intend to have the characters find out that this deceased character was actually his little sister. Now, I don't think this will change much of the ending of the book, but I still think it would be an interesting twist to the story and it would definitely change his view of the character. Neither character involved in the twist is the protagonist of the story, though you do see things from the potential brother's perspective twice.

I'm not asking whether or not this specific twist is good, rather, is it needed? In a more general sense, how do you differentiate between a good plot twist and a unnecessary page filler?

Answer

Plots twist. Stories don't. Sometimes a plot twist will show that reader that the arc of the story was something other than what they were expecting. But the story still needs to have a satisfying arc. If the plot twist destroys the story arc, or is simply irrelevant to the story arc, then it ruins the story. If the plot twist is integral to the story arc -- and generally that means the moral arc of the story, it is essential.

So, a plot twist is not an optional extra that you throw in gratuitously at the end. It is either where the story was heading all along, and the thing that reveals the story for what it is, or it is a disastrous distraction that destroys the story arc.

That's how you tell the difference. Either the twist is necessary to the story or it is superfluous. Either leaving it out ruins the story or putting it in ruins the story.