If There Be Thorns (Dollanganger #3) by V.C. Andrews

 Chris and Cathy are raising, Cathy's children the third installment of the Dollanganger series.  Jory is a teenager who is following the footsteps of his biological father and his mother, Cathy as a dancer for the ballet.  He is handsome, talented and coming of age. Bart is young boy who is handsome but peculiar and clumsy. 

Life is quite pleasant until the empty house in disrepair next door becomes occupied. The new mistress of the house uses her riches to win over Bart however the servant in the home gives Bart a diary to read and fills his mind with stories about sin and the sins of women. 

Bart undergoes a transformation and becomes hateful, violent, destructive and disobedient.  Things get worse when Cindy becomes a part of the family. Chaos and darkness ensues and Cathy will have to fight to win Bart back. 

This is an easy read that delves deeper into the Dollanganger series. Lots of drama.