Repetition

Names are also repeated or very similar e.gLogan(surname in Logan series and first name of Heavens husband in Casteel series), Cary/Cory(Cousin/husband of Melody in Logan series and brother of Cathy in Dollanganger series - also both characters are twins), Catherine (main character of Dollanganger series and friend of Pearl in Landry series and grandmother of Ruby in Landry series), Jack (grandfather of Ruby in Landry series and lover of Pearl from Landry series... also the doctor who trys it on with Pearl is also called Jack), Olivia (grandmother in Dollanganger and Logan series)

Twins are also repeated throughout eg Cary and Laura(Logan series), Cory and Carrie(Dollanganger series), Ruby and Gisselle(Landry series), Pierre and Jean(Landry series),Darren and Deirdre(Dollanganger series)

Another factor of the twin one is that usually one of the twins in almost every set dies young eg. Jean, Gisselle, Laura, Cory

I'm sure you can think of many more examples for these as well.

Unusual names are also used reguarly eg. Pearl, Ruby, Gisselle, Pierre, Cory, Carrie, Jory, Bart, Melody, Rain, Summer, Cary, Sara (spelling unusual), Haille, Beau... theres loads more here too

How the books are laid out are also repeated e.g 1st book - main story that sets the basis for the storyline and rest of the books.
2nd book - main character had love interest and falls pregnant
3rd book - main character has child that is taken away from them at some point for a short while, marries main love interest and lives a good, happy life
4th book - from child born in 2nd/3rd book point of view, some big event happens that changes their lives, and have love interest
5th book - from point of view of mother or grandmother of main character and this book usually explains why and how events in the 1st book came about

The characters who narrate the book are generally female, and the main love interest is usually a year or two older than them.

A grandmother (whether nice or not nice) has a big influence on the main character and usually appears mainly in the 1st book or first 3 books of the series.

Also, there is a mother/stepmother who is not nice and doesn't see whats happening right under her nose.

Incest usually appears or relationship with someone who the main character believed to be relative and found out isn't. Also, incest before couple actually find out are related or never do find out and their child does after their deaths or we find out in the 5th book.

Rape is frequent as well. Often, a child is produced out of that. Usually the rapist will be a relative of some kind to the main character.

The main characters in almost every book are usually considered 'the prettiest girl in school' and very beautiful whilst very shy, naive and innocent... their main love interest generally tends to be very handsome and compared to movie stars and models.

There is also usually a character that is promiscuous or extremely outgoing and won't say no to alcohol and drugs etc. Usually this character is the mother or sister of the main character.

Another storyline repetition is the main character starts off to be poor in the 1st book and goes on a journey of some kind during that book and ends up living somewhere very rich in a big house by the end of the 3rd book.