Is this really a true story?

FFS - It's a ghost story, of course it's not real.

In case you were wondering The Santa Clause isn't real either.

Is it real? FFS.

Well considering there's no such thing as any of this, no of course it's not real.

It's about as factual and based on real facts as Joy was about Joy Mangano's real life.

In Joy, there was indeed a woman who invented a Mop. In Real life there was a woman who invented a mop. That's about all the similarity there is to the movie and the real story.

Same here. There's a purported haunted house in England. The Warrens were alive at the same time it happened.

The Warrens themselves were real people and their stories are (apparently) true, though obviously the movies are obviously heavily dramatized. Plus, the Warrens themselves have been accused of being frauds. Can't say for sure personally unless you were there.
https://www.youtube.com/watch?v=uiog10FEGA4

Whenever you see the disclaimer "Inspired by true events" at the start of a movie, you can be sure almost all of what follows is fiction. Likely only the basic premise has any relation to reality.

Whenever you see the disclaimer "Based on true events" at the start of a movie, you can be sure a lot of what follows is fiction. Characters are added, plot and time lines fit to make the movie more interesting, etc.

As for this movie, do you really believe there are demons, and that they can be banished simply by invoking the name of God? I would hope no one believes that. But hey, people can convince themselves anything is true.

I'm related to a friend of one of the household members, and heard a little about it first-hand. It happened, but nothing like the movie. In real life, though, it doesn't have to be like that to be terrifying.

The real interview with the sisters is on youtube: https://www.youtube.com/watch?v=sls6pszMGfk
She is speaking in that mysterious voice, but the movie overdid it for effect.