The Malfoy/Black/Lestrange with an exception of the Tonks family is very messed up.
Bellatrix killed Sirius even though they're cousins. She also killed Tonks, and Tonks is her niece.
The Blcak family are very strict pure-bloods, so when Andromeda married a muggle-born(Edward Tonks), they disowned her and her named was burned off of the Black family tapestry.
I don't know how Narcissa felt in this situation, but Bellatrix never exactly liked Tonks, which lead to her trying(and sadly suceeding ) to kill Tonks. (this is why I always make fun of Bellatrix. she kills all of my favorites, like Sirius, Tonks, and Dobby.)
Draco was threatened that if he didn't become a Death Eater, him and his family would die, so he joined. His family includes Narcissa, Lucius, Tonks, Sirius, Bellatrix, Rudolphus, Andromeda, and any other people in the Malfoy/Black/Lestrange/Tonks family. Something I realized: Draco was threatened to be killed along with his family if he didn't become a Death Eater. Bellatrix Lestrange is his aunt, which makes her a part of his family. But I don't think Voldemort would ever kill her, due to the fact she's his most loyal servant. So if Draco didn't become a Death Eater, I don't think Bellatrix's life would've been in danger, I think it just would've been his mom, dad, and a few cousins, like Tonks, Andromeda, and Sirius.
This is why the Malfoy/Black/Lestrange with and exception of the Tonks family is very messed up. (sorry about this random rant.)