I forgot the name of this book series?

ok, i read it 4 years ago and it was a myster/ creepy book series. The author would write creepy novels which don't relate with each other but are similar in "creppyness" or 'weirdness". i remember this one book from her collection where the main character is a girl who is 17 years old and her dad is getting engaged to another woman who also has children. That woman is very strict and her children obey her. as the plot goes on the main character finds out that the "about be Mom and siblings" which she is abut to be connected with don't grow or aren't affected by time. that means that the 14 year old daughter, the woman and the son dont age. I know its weird but it was very pleasing to read. Does anyone know the novel or author, its driving me crazy!