The movie I'm trying to remember was seen in the early-mid 90's. I believe it was a made for tv movie (Lifetime, maybe USA or some other channel). Anyways, the basic plot was:
A family (mother/wife, husband/father, & their young son) either take in or higher this attractive young blonde girl (who I could've sworn was Alicia Silverstone, but I've been through her IMDB & wiki page thoroughly & it's not her) as a sitter and/or care taker for their home. As the movie progresses the wife becomes suspicious of the young girl & her husband developing a relationship, even her son is strangely attached to the young girl & he starts drawing weird pictures depicting demons, saying they're inspired by the young girl. Eventually the wife meets a man who reveals that he is the young girls father & that she is highly disturbed, very bad news, & that her family is in danger. He teams up with the wife in hopes that his daughter will be put away. I believe later in the movie (possibly immediately after the meeting between the young girls father & the wife) the young girl kills her father, or at least tries to, by running him over (I think). Anyways, towards the climax/end of the movie, the husband & son are performing some demonic ritual (possibly involving the murder of the mother/marriage to the young girl). I don't remember exactly what happens after that, but the family snaps out of it (I wanna say they think she died somehow during a mishap at the failed ritual) & things go back to normal, happily ever after, yada-yada-yada. At the very end of the movie, the back view of a young blonde girl is seen holding her thumb out on the side of a road during a dark night. A car holding a family (husband, wife, 2 young kids) stops & picks her up. The movie ends with the girl climbing into the back seat, next to the 2 kids, looking very grateful & happy to have been picked up by the family. BUT WAIT! DUN DUN DUN! It's the evil young girl (only with dorky "innocent" glasses).
There's also this one particular scene that I remember vividly, the wife & husband are being intimate in their bedroom & there is a moment when the wife looks towards a mirror that is in the room. When she does this, instead of seeing her & her husbands reflection in the mirror, she sees her husband & the young girl. I want to say this particular scene is in black & white, and possibly a bad (day?)dream that the wife was having.
If anyone has any information about this movie, please let me know. I only saw this movie on tv once & I loved it. I've never forgotten it & it drives me crazy that I can't remember it's name.