The Malfoys are related to the Black family through Narcissa (a first cousin of Sirius Black, Harry's godfather), which makes Draco a nephew of both Bellatrix Lestrange and Andromeda Tonks.
As pointed out, Sirius Black was actually Draco's first cousin once removed - not his uncle. That being said, according to J.K. Rowling, yes, Harry Potter and Draco Malfoy are related - albeit distantly - by marriage, if not by blood.
Bellatrix Lestrange, Narcissa Malfoy, and Sirius Black are all descendants of Pollux Black, making them first cousins. Note: For space's sake, this tree doesn't include Andromeda Tonks or her husband and daughter. Sirius had good reason to avoid his relatives, as many in his family served as Death Eaters.
Bellatrix Lestrange (née Bellatrix Black) is a fictional character of the Harry Potter series and is the sister of Narcissa Malfoy, the wife of Lucius Malfoy, and Draco Malfoy's aunt. Her parents are Cygnus Black III and Druella Black (née Rosier) and the eldest of three sisters: Bellatrix, Andromeda and Narcissa.
Delphini was born in secret at Malfoy Manor in early March 1998 as the result of a liaison between Bellatrix Lestrange and Lord Voldemort. On 2 May 1998, both of her biological parents were killed in the Battle of Hogwarts.
How EVERY Harry Potter Character is RELATED (All Family Trees Explained)
Did Narcissa divorce Lucius?
When Lucius turns on Voldemort due to the reveal of his Horcruxes, Narcissa divorces him, taking 75% of Malfoy's wealth with her. Even her relationship with Draco is different, with POS-Narcissa only viewing Draco as a tool to further her own agenda.
Based on what Cursed Child lays out, Voldemort would become Draco's uncle (pretending he had married Bellatrix), thus Scorpius' great-uncle, making Delphi be Scorpius' cousin-once-removed.
The pure-blood wizards and witches featured in the Harry Potter books are almost all supremacists, while there are some of them who do not advocate ancestral superiority; the Potters, Weasleys, and Longbottoms are old pure-blood families, but no known members of these families are sympathetic to supremacist aims.
Black did not have a wife or children. As the portrait of Phineas Nigellus, Black's deceased great-great-grandfather, stated, the direct line of the ancient Black family ended with Sirius' death. Had Regulus Black lived, he would have been the heir of the Black home; however, he predeceased Sirius.
Remus fought at the Battle of Hogwarts on 2 May 1998, during which his wife was murdered by Bellatrix Lestrange. Remus was also murdered by Death Eater Antonin Dolohov, during the first half of the same battle. His death was possibly avenged by Filius Flitwick.
And Remus was in love with Sirius (also hopelessly, because Sirius only had eyes for James). Then James was killed, Sirius was imprisoned, Remus was outcasted, and Peter was ratatouilled.
Persephone Malfoy, though not an inherently evil woman, is nonetheless a deeply flawed woman, albeit not of her own doing. Her flaws, however, served to contribute to a rather grim and unhappy childhood for Lucius, who was an only child.
Cedrella and Septimus went on to have three children together, one of whom was Arthur Weasley – Ron's father. This means that Draco and Ron are distantly related, making Draco's bigoted pure-blooded ideology even more ironic.
And while showing Harry the family tree, Sirius says he is related to the weasleys. Molly and I are cousins by marriage and Arthur's something like my second cousin once removed. So Sirius is related to Ginny and so is Malfoy.
Harry Potter had a few love interests during his time at Hogwarts. Though he eventually settled down with Ginny Weasley, he also had a thing for Cho Chang in his earlier school years. Then, of course, there's his secret love for Draco Malfoy…
In the books, Pansy Parkinson has a schoolgirl crush on Draco, and they went together to the Yule Ball. However, there was no true love between them. In the Cursed Child, Draco Malfoy's wife is Astoria Greengrass.
During this year, Narcissa became pregnant and miscarried. She was distraught and uncertain of what to tell Lucius, as she had not told him of her pregnancy. Shortly thereafter, he brought her out to the Leaky Cauldron to visit with his friends Vettlingar Yaxley and Thorfinn Rowle.
She married Rodolphus Lestrange after leaving Hogwarts "because it was expected of her" to marry a pure-blood. However, Rowling stated in an interview that Bellatrix truly loved Voldemort.
Narcissa, the youngest of all three, made a respectable marriage after graduating Hogwarts, and into another pure-blood family. It was Lucius Malfoy, a Death Eater, who brought Narcissa into the darker side of the wizarding world – but tellingly, Narcissa never became a Death Eater herself.
