I'm doing this one from memory as well, so please bear with me. If I get anything wrong, just let me know and I will amend it below my original review.

If There Be Thorns is the third book in the Dollanganger series. The story continues with the incestuous siblings, Cathy and Christopher Sheffield. However, the focus is now shifted to half-brothers Jory and Bart Sheffield, Cathy’s two boys.
Cathy and Christopher Sheffield raise the boys together even though she feels that one day their dirty secret will be revealed to them, causing yet another rift in the family.
The ‘skeletons in the closet’ continue as Bart keeps his new friends, Corinne and her butler, a secret from his family. That secret is finally revealed and dealt with in the usual ‘Sheffield’ fashion.
Cathy, still bitter and bent on revenge, pops over to her mother’s house to confront her. Corrine expresses remorse for her actions and begs forgiveness. Bart, still under the influence of the butler, locks his mother and grandmother in his grandmother’s cellar. Despite their sins, Bart realizes he still loves them. He reveals their location to Chris, but not before the house catches fire. Cathy is saved but her mother dies.
You can continue to follow their family saga as it continues with Seeds of Yesterday.