There are so many different facets to the world of Harry Potter that the fandom can often fixate on characters or time periods that are ancillary to the main plot. There are parts of the fandom that mainly care about the Marauders, for example, or the founders of Hogwarts. There is also a portion of Potterheads that are really interested in the backstories of the three Black sisters - Narcissa, Bellatrix, and Andromeda. Those names are likely familiar to those who know a thing or two about Harry Potter, but not everyone knows that they're sisters, or how drastically their lives diverged.
The three girls grew up in the infamous Black family, one of the most notable pureblood wizard families in the wizarding world. Sirius Black, part of the same family, was their cousin, which is a piece of information that comes up a few times in the series. All three women married into other families, and two of them even joined Voldemort's ranks. Bits and pieces of their backstories have been revealed or hinted at, which has led many fans to extrapolate and discuss further what the sisters' lives were like. If they grew up together, how did they all wind up on such different paths?
Narcissa Malfoy
Narcissa was the baby of the family, the youngest daughter of Cygnus and Druella Black. Like many pureblooded wizards, the Black girls were taught the importance of blood purity and told that they were superior to the wizards who were half-bloods or Muggleborn. They certainly had a disdain for Muggles as well. When Narcissa went to school at Hogwarts, she was sorted into Slytherin, where she met Lucius Malfoy, whom she would later marry. Lucius came from a family that was just as wealthy and pure-blooded as her own, so it was seen as a perfect match.
During the time of the First Wizarding War, Lucius introduced Narcissa to the activities of the Death Eaters (who were also focused on blood purity), though she never became one herself. At the end of the war, Lucius was able to avoid being sent to Azkaban for his involvement with the wrong side by claiming that he had been under the effects of the Imperius Curse. It probably didn't hurt that the Malfoys were close with a lot of Ministry of Magic officials, including the Minister for Magic himself. Lucius and Narcissa also of course, had a child together - Draco.
Narcissa wanted to keep Draco safe, which becomes an important thread throughout the Harry Potter series. During the Second Wizarding War, Lucius wasn't able to escape the fate of being sent to Azkaban for being a Death Eater, so Narcissa tasked Severus Snape with protecting Draco on his mission (being sent by the Death Eaters) to kill Dumbledore. During the Battle of Hogwarts, Voldemort attempted to kill Harry, and sent Narcissa to check if he was dead. She realized he wasn't, but quietly asked if Draco was alive. When Harry answered that he was, Narcissa told Voldemort that Harry had died, saving him. She abandoned the Death Eaters altogether during the battle as she and Lucius searched for Draco. Because they turned from Voldemort, the family was granted a pardon after the war and they weren't sent to Azkaban.
Bellatrix Lestrange
Bellatrix is one of the more memorable characters from the series, as she has a distinct style and personality, often acting a bit scary and unpredictable. Bellatrix was the oldest of the three sisters, born in 1951. When she went to Hogwarts, she was also sorted into Slytherin. She believed heavily in what she had been taught about blood status, and looked down on anyone who was not loyal to their family in that regard, including her own cousin (Sirius) and her sister Andromeda (who had very different beliefs on the subject that will be discussed later on). Bellatrix married Rodolphus Lestrange, who was a wealthy, pure-blood wizard, despite seemingly not feeling affection for him at all. They had no children together.
The Lestranges joined Voldemort's ranks when he first rose to power, and Bellatrix became fanatically loyal to the Dark Lord. At the end of the First Wizarding War, Bellatrix was sent to Azkaban for her role as a Death Eater. She spent 15 years there before her escape, which took a toll on her mentally (as is evident throughout her appearances in the Harry Potter series). She fought for Voldemort once again during the Second Wizarding War, committing numerous heinous actions along the way, including torturing Hermione Granger, as well as murdering members of her own family, Sirius Black and Nymphadora Tonks (though she considered them both blood traitors and felt no loyalty toward them). She was finally killed by Molly Weasley during the Battle of Hogwarts, bringing an end to her lifelong fight for Voldemort's reign.
Andromeda Tonks
Andromeda was the middle child, and ended up being the black sheep of the Black family (or, one of them, at least). It seems that the family's views on blood purity did not resonate as much with Andromeda as they did with her sisters, as she had more of an open mind. She claimed that Sirius was always her favorite cousin, and she appreciated him when other members of the family did not. Andromeda was also a Slytherin, like her sisters, but this was where her path began to diverge from the rest of her family. At some point, either during her time at Hogwarts or after (it's unclear), she met and fell in love with Ted Tonks, a Muggle-born wizard. Her entire family disowned her because of this, and she became a blood traitor to them, having no contact with her sisters for years.
Andromeda and Ted had one child together, Nymphadora, who is well-known to fans of the Harry Potter series as simply Tonks. Andromeda and Ted were not officially part of the Order of the Phoenix, but they still allowed their home to be used as a safe haven for the organization. When the Ministry of Magic fell to Voldemort, the safe houses were no longer protected as Andromeda and Ted were tortured for information about the Order. They survived this ordeal (without giving too much away), and continued to help the Order. Unfortunately, Ted was killed by Snatchers during the war, leaving Andromeda widowed. She also had to endure the loss of her daughter after she was killed by Andromeda's own sister, Bellatrix. Andromeda made it out of the conflict and ended up raising her grandson Teddy after he was orphaned by the war. It seems that her story had one of the sadder endings, made even more tragic by how she tried to fight for good during her life.