We get a flashback to her past/what leads up to this moment in a later chapter.
tl;dr is her family is absolute trash
Dad is a spineless guy who do whatever stepmom say
Stepmom is a bitch who drinks all day/whine all the time but never do anything to help
And two younger brothers who doesn't listen to her
She was initially selling flowers to make some money, then one day get recognized as having Saint power and thus enter the school.
Lana stuffs happen.
She was getting gifts from the prince after the Lana stuff so she was happy for a bit.
until one day she came home to find all of her gifted dresses gone
the bitch stepmom had sold all of them for booze money
Sandra went to cry in her room about how miserable her life is
Then she smelled something burning
Went to check, to see the brothers crying for help after having started a fire from playing with matches
The parents? drunk asleep on the couch
At first she was about to step in, then pause, them mutter to herself "I saw nothing, I went out for a walk and when I came back everything's on fire..."
And close the door, went out to wash herself from any smell of soot, and go for a walk
Soon enough someone came to tell her that her family house is on fire
Then as she 'cried' in Evan's arms we get to see the real 'smile' she made, one of a broken person, laughing in madness (something like "There's no going back anymore, I WILL find happiness, NO MATTER THE COST")