The relationship between Danielle and Aster is a complex and layered one. They are paternal half-sister via their father, Mason Saybrook. However, both were unaware of this fact until they were much older, although, Danielle found out five-years before Aster did.
The girls met for the first time when they were very young, with Aster being slightly older than Danielle. They were fast-friends and often hung out at the Meriweather Estate, as the Gilchrist family were caretakers there. They lived in a house on the island, just away from the Saybrook estate. As they grew, they became closer until they eventually became best-friends. In their teen years, Aster gave Danielle a Saybrook diamond tennis bracelet as a birthday present, courtesy of Saybrook's Diamonds. They also started being misconceived as sisters due to their close-knit relationship and identical blue eyes. This was a popular misconception that they played along with and used to their advantage while they were shopping or in bars.
The summer prior to Steven Barnett's murder, Aster began to suspect that something was off with Danielle. She believed that Danielle was seeing someone, although, Danielle never told her who. She then noticed odd behavior between Danielle and Mason, Aster's father. Simultaneously, she started to suspect that Mason was having an affair, though she wasn't sure with whom. One day, while the girls were at the pool, she caught Mason staring at Danielle who was wearing a revealing bikini. This launched Aster to believe that Mason was having an affair - but with Danielle. She watched them for a few weeks and saw them engaging in intimate glances and caring hugs. These incidents confirmed Aster's suspicions of Danielle having an affair with her father and she cut Danielle out of her life completely.
The girls were reunited at Steven Barnett's funeral, where Aster refused to talk or even look at Danielle. She also noticed that Danielle was wearing the tennis bracelet that Aster had given her as a birthday present. Aster made a snide remark towards both Danielle and her mother, Julia, and the Gilchrist's went to the other side of the church and left the Saybrook's alone to grieve as a family.
Five-years-later, Aster was cut off from her allowance by her father, and given a job at Saybrook's Diamonds, which she despised due to her privileged outlook on life. On her first day, she was given a new-employee presentation by a surprised Danielle. Aster remained cold and distant, never speaking to her unless she had too. When PoppyPoppy died, Danielle attended the funeral and gave her condolences to Aster and the other heiresses. She then, once again, gave them their space to grieve as a family. At Saybrook's Diamonds, Danielle gave Aster her sympathies over Poppy's death, stating that she always liked Poppy.
Weeks later, Danielle and her mother gave the heiresses a ride after their car wouldn't start. The heiresses were scared, and begged Danielle to just drive and take them back to the mainland, but didn't tell her that they feared they knew who really killed Poppy - F.B.I. agent Katherine Foley, who was also the lead investigator in Poppy's case.
Aster then remembered that Danielle had also been on the security footage from the night Poppy died, and questioned her about it due to Danielle having claimed earlier that she was sick at home with food poisoning. Danielle reiterated that she had been at home due to eating bad sushi, but Aster continued to grill her, revealing her suspicions that Danielle had killed Poppy to get back at Mason. She believed this to be the truth, and once they reached the bridge the cousins had almost died on weeks prior, she demanded that Danielle stop the car at that very moment. Danielle slammed on the breaks and stopped as Aster had asked her too.
Aster started to get out of the car when Julia commanded that she stop, whilst holding a gun pointed straight at Aster. She then told all of the girls to get out of the car very slowly and held them all at gunpoint on the bridge. Julia then all-but told Aster the truth of Danielle, and Aster figured out that Danielle was really her sister. That over the summer, Danielle and Mason weren't having an affair, but rather were growing closer as father and daughter. Julia was the one who had an affair with Mason, and the affair resulted in the birth of Danielle.
However, Julia was infuriated with Mason for hiding Danielle away from her sisters and cousins, as well as robbing her of her birthright of being an heiress. Danielle had become aware of her true father during that summer, and that was what she and Mason whispered about. Aster was shocked to learn that Danielle was her sister, and when she feared that Julia was going to kill them all, she grabbed Danielle's hand and attempted to apologize via a single, yet firm, hand squeeze. they were rescued by the F.B.I. and Julia was believed to have been shot and killed, though her body was never found.
In the weeks that followed, Danielle distanced herself from the Saybrooks as they were all reeling with the many revelations that Julia delivered. One of the revelations being that Mason had a secret daughter and that both Corinne and Aster had another sister. In a confrontation with her father, Aster demanded that Danielle be a Saybrook in every way now - including being an Heiress to the family's fortune. Mason initially wasn't happy to oblige but agreed to follow his youngest daughter's wishes none-the-less.
A year later, Aster had fully accepted her as her sister and even invited her to live in the Meriweather Estate as an official resident. Danielle also forgave Aster for cutting her out of her life and assuming the worst of her. Aster also helped Danielle become an heiress to the company, and boost her social status. While both women had a lot to work on as far as trust went, they were headed in the right direction by the end of the novel.
- They were best friends when they younger, and throughout their teen years.
- They used to pretend they were sisters when they would go to clubs, just to put on an "act".
- Danielle taught Aster a lot about boys and drinking, and the two would party often.
- Danielle found out that Mason was her biological father five-years before Aster did, and promised Mason that she wouldn't tell Aster.
- Due to Danielle and Mason's closeness, Aster believed they were having an affair and completely cut Danielle off. This hurt Danielle, although, she never confronted Aster due to her promise to Mason.
- They are paternal half-sisters, due to sharing the same father but different mothers.