Reason for relationship change?

The relationship that you described in the novella sounds much more complicated and confusing. The one we saw in the movie is pretty straightforward and already comes with a lot of built in personalities like the evil stepmother and the naive father and the noble daughter.

Movies have much less time to explain things compared to what books can do, so maybe they made it simpler to save time and allow them to focus on other things? If Kristy was this random friend with feelings for another woman's husband, that automatically associates her with a villain role, and the movie would have to take at least some time building up her character more to make her into the hero again. If theres no benefit to the more complicated relationship, why not save the trouble and make things easier? If the audience is unnecessarily confused over minor details, its not an effective story.

I initially thought the same about it being overly complicated. My boyfriend read the story before I did, and when he described the original relationship, I assumed it was too much detail to include in a film adaptation. After reading it, however, I'm confident it could have easily been done. The original story is a novella-barely longer than a short story- so it's not as if Barker had tons of time for character development, but the friendship was established without complication. For the movie, it could have been as simple as something like:
LARRY: Oh, Kirsty's stopping by to help unpack.
JULIA: (sarcastically) How generous.
LARRY: Can't you be a little nicer? She's our friend.
JULIA: She's YOUR friend. And if you think she's only interested in your friendship, you're being naive.
...Or, you know, something slightly less cheesy an actual screenwriter could come up with, haha. Pair that brief dialogue with Kirsty acting overly eager around Larry and uncomfortable around Julia, and it would be a quick, easy set-up.

That said, I do agree that the whole "evil stepmother" concept adds a much more interesting dynamic than the friendship in the novella had. I'm just curious whether Clive Barker ever specifically mentioned why he changed it since it was HIS idea in the first place- if he just wanted to improve upon his original story by changing the relationship, or if it was pressure from the studio or something.