The Seven Sisters are a set of... well, seven sisters who are highly important to the Forgotten Realms. At least, they were prior to Dungeons & Dragons 4th Edition. They are the result of the goddess Mystra specially engineering the birth of seven girl-children from a possessed mortal host to serve as her Chosen, since she'd had very bad luck with picking adults to fill the role, as they tended to either go crazy or be more independently minded than Mystra wanted. Elminster played a big role in their lives.
Mystra possessed a she-wizard named Elué and seduced a ranger named Dornal Silverhand, whom had been in love with Elué, to conceive seven children in turn. But a mortal host couldn't sustain a god's essence for the years it took, and towards the end of the 7th and final pregnancy, Elué's body was basically a spectacularly well-preserved lich. Dornal grew suspicious of why his wife was so weird and managed to figure out that she had been possessed; confronting her, he forced Mystra to reveal the truth, and was so disgusted the revelation that he turned his back on both arcane magic and his seven daughters, walking away and avoiding them for the rest of his life.
Anastra Syluné Silverhand
More commonly known as "Syluné", she was a founder of the Harpers and spent many years as the kindly Witch of Shadowdale before being slain by a Red Dragon. She existed as a unique ghost called a Spectral Harpist for a long time after that, but was ultimately destroyed by agents of Shar.
The Lady of Silverymoon, founder of the nation known as the Silver Marches.
A towering giantess of a woman who, ironically, disdained her natural talent for magic and instead preferred to focus on swordplay and wilderness lore; she served as a ranger in an adventuring company known as the Knights of Myth Drannor, marrying a fellow Knight named Florin Falconhand.
Laeral Silverhand Arunsun
Married the wizard-lord Khelben "Blackstaff" Arunsun of Waterdeep and became his beloved consort. Spent a long time being mind-controlled by an evil crown possessed by a sliver of a dead god.
Birth name "Alassra Silverhand". Raised by Elminster, grew up to become his lover. Founder of the kingdom of Aglarond, a self-titled "friend of the fey" and an interfering self-righteous busybody who secretly dreamed of shattering all human nations into small, rural communes that would be forced to live in harmony with the fey and demihuman races. Incredibly powerful, more than a little mad, and a bitter enemy of Thay.
Arguably the most Mary Sue of all the Seven Sisters. When Mystra was forced to reveal herself to Dornal Silverhand, it proved more than her host-body could stand. Desperate to save her seventh daughter, she made a bargain with a pregnant drow priestess of Eilistraee who had led a congregation of renegade drow away from the major cities of the Underdark, inadvertently killing her own baby in the womb through the stress. With the agreement of the priestess and Eilistraee, Mystra swapped the drow's dead child for her own living one, and Qilué was thusly born as a drow who grew up to be both a powerful wizard and a priestess of both Mystra and Eilistraee simultaneously.