This is one of those instances where I disagree with the majority of critics. I've always liked this film. Louise Fletcher [Firestarter, Cruel Intentions] is a delight to watch as the uber-oppressed, black dress wearing, Bible-toting grandmother. She doesn't even have to say a word. Her mere appearance is creepy enough to unnerve anyone. Victoria Tennant is also very good as the mother who is absolutely clueless as to what it happening with her children and couldn't seem to care less.
The real stand out among the kids is Kristy Swanson [Buffy the Vampire Slayer, Ferris Bueller's Day Off] as Cathy. She reminds me of a young Patricia Arquette and fills the role with passion and true emotion. Jeb Stuart Adams [The Goonies, Once Bitten] plays her brother, Chris, and he is also quite good.
One of the drawbacks in this film is the make up used on the children as they get sicker and sicker. They sort of look like the Cullen family from Twilight which, for me, is not a good thing. A little subtler approach would have been more effective.
Flowers in the Attic is the kind of film that's perfect for a rainy Sunday afternoon. There are plenty of films out there that will scare the b-Jesus out of you but this one is just plain creepy and is able to keep us that feeling for the entire length of the movie. If you've never seen this one, it really is worth a try.
For more info check out the film's entry in IMDB.