Here's why:
* Bellatrix is Draco's aunt by marriage: She is the sister of Lucius Malfoy, Draco's father. This means she is related to Draco through his father's side of the family.
* Harry's lineage is entirely separate: Harry is related to the Potters and the Black family, but there's no connection to the Malfoys or Lestranges.
Therefore, despite Bellatrix's connection to both Harry and Draco, they are not related.