Let's see if I can be twice lucky today. I was given a children's or YA horror novel sometime in the '90s, which I actually never read as I found the premise too frightening at the time! Future Dan and his love for Lovecraftian horror scoffs at his past self. Here's what I remember:
I believe the book cover featured a looming cathedral, with a robed and hooded figure stood outside; from the robes, one could see a tentacle where the figure's arm should be.
The plot summary included a brother and sister; I think one of them fell under the sway of the evil force or cult in this cathedral, and their sibling had to save them.
That's all I've got. Let's see if anything hits!