In the great hall, the Malfoy family demonstrated themselves
as peaceful and not with the death eaters--- but just a family
seeking a chance to catch their breath.
For a while, no one
cared for them, too busy with their dead and mourning. That is
until they were recognized by someone holding a grudge and
started something. There was bickering back and forth and an
auror stepped in and decided to take away Lucius. A student
then shouted something about Draco being a death eater.
The man then remembered the papers saying he was spotted
in the death eater group that killed Dumbedore, so Draco was
forced to show his arm in front of the auror and some students,
revealing a dark mark.
So Draco had to go with them and Narcissa followed them silently,
holding Draco's hand and trying to get to lucius, but the auror was
blocking her---trusting the woman and child behind him and lucius
in front (after all, he did escape Azkaban, he is already charged
with crimes).
Since they have nothing on Narcissa but probable
cause, they would keep her at house arrest until they realized how
awful the malfoy manor had gotten during the war so they had her
stay with a friend with 24/7 auror parole.
Draco and Lucius, however,
went straight to Azkaban because they both had the dark mark,
signifying them as traitors to the wizardry world. They asked for their
wands but when they said how both of theirs had already been taken
from them, they felt like shit--- but that actually helped them because
they couldn't check the spells they used.
Lucius and Draco were not
cellmates --- Draco went somewhere where there was juvenile
(he was 17, under 18) and Lucius went with the big boys and back
to his original cell
Then they were both called for trials and Lucius had to stay in prison
to serve his original time and Draco was sentenced for a summer and
then watched as he completed his last year at hogwarts.
They dont have any evidence lucius, narcissa, or draco did anything
bad. Harry won't say because he owes narcissa for his life and that's
how he repays her--- her familys freedom as much as he can give her
