I know I watched this around 1999-2003 because I remember watching it in my studio apartment in Ohio as a DVD checked out from the library. I think it was an older film, considered a classic (although I remember it seeming somewhat hackneyed at the time). The concept involved a malign entity (a ghost? a demon?) that was killing various characters.
The scene that stuck out in my mind has a young woman, the character likely a late teenager, taking a shower at the family home. I think there was a scene earlier where her younger brother (probably prepubescent) pulled a juvenile "prank" on her, I think just jumping out at her to startle her. I don't think the girl knew that she was a target of the entity. Anyhow, after the shower (I think with only silhouetted nudity behind the shower curtain), she steps out of the shower and to the mirror, at which point the entity takes control, forcing her to unfold a straight razor and cut her own throat, all while her eyes and general face showed her terror as she tried to regain control.
Right afterwards, I think as she was collapsing, her younger brother suddenly stuck his head through the open window and yelled something like "Surprise!" before the window frame suddenly fell and struck him on the neck, seemingly breaking his neck. The suddenness of it has stuck with me over the years. Both victims were Caucasian. I don't think either one was blond-haired. The brother might have had red hair. I'm pretty sure they were at home with their parents, because I have a vague memory of a later scene having the parents find the body, and their grief at what seems like it may have been a tragic accident.
The only other scene I recall with any clarity is one involving two people in a car parked by a body of water (I want to say a lake or a river). These two know that the entity is after them, but think they're safe, only for one to suddenly attack the other as the camera pans back to show that there isn't anyone near enough by to investigate the car as the muffled sounds of the kill happen within.