i read the book too, about three times and i agree with you

I was disappointed in this film, but I'm very tempted to seek out the novel because there's a very compelling story here. But the movie is pretty clumsy, like a cheapo TV or cable production.
The plot points you mentioned don't bother me as much (you do mean changing it from the grandmother to the sister for the movie, right?), but the execution is just bland. Unfortunately, that's the case of a lot of horror films. They work better as books because your imagination provides the imagery.

I did not care for the movie but I may buy the book since so many are saying the book is better.