Sally Hemings was enslaved because her mother was enslaved
Sally Hemings was seven-eighths white, and the half sister of Jefferson's wife, but under Virginia law she was still a slave.

The Rest Is History · The Man Who Really Founded America: Thomas Jefferson | Founding Fathers EP 4Sally Hemings was the half sister of Thomas Jefferson's wife, and she was also their legal property. That strange fact comes down to one rule.
Hemings's family history is unusually well recorded because her son Madison Hemings, who was enslaved at Jefferson's hilltop plantation home, Monticello, later wrote it down. Her grandmother was a full-blooded African woman who was forced into pregnancy by a captain from England called Hemings, from whom the family took its surname. Their daughter, Betty Hemings, was therefore half white and half Black.
Betty came into the possession of John Wayles, a planter from Lancashire who also bought and sold enslaved people and chased unpaid debts. Planters in debt feared his arrival. Tobacco prices had fallen sharply, leaving many Virginia plantations in debt. Wayles kept Betty as his concubine and fathered six children with her. The last, Sally, was born in 1773. She was counted as seven-eighths white by the racial terms of the time.
Under Virginia law, that made no difference. Virginia law borrowed from ancient Rome the rule that a child's legal status came from the mother, not the father. A child born to an enslaved woman was enslaved, even if the father was her white owner.
When Wayles died in 1773, his enslaved people passed to his heir. That heir was his only surviving legitimate child, Martha, known as Patty, who had married Thomas Jefferson in 1772. The six children Wayles had fathered with Betty, Sally among them, thus became the property of their own half sister, and then part of Jefferson's household. Sally served Patty's daughter as a maid, and was aunt by blood to Jefferson's own daughters.
Questions people ask
Why did Virginia make a child's status follow the mother?
Yes. In Virginia, and later throughout the slave states, children inherited the legal condition of their mother. The rule was adopted in Virginia in 1662, reversing the English rule that a child followed the father, and lawmakers kept it because it increased the number of enslaved people and protected white fathers from claims by their mixed-race children.
Was Sally Hemings related to Jefferson's wife?
No. Jefferson married the widow Martha Wayles in 1772, and she died in 1782. Sally Hemings was Martha's much younger half sister, the daughter Martha's father had with the enslaved Betty Hemings. Hemings later served Jefferson's daughter and then Jefferson himself as a chamber servant.
Did Sally Hemings ever go free?
She remained enslaved in law for life, though Jefferson's daughter gave her informal freedom after his death. Jefferson freed all four of her surviving children with him, two in his will and two earlier by allowing them to leave Monticello. She died in Charlottesville in 1835.