The manga version doesn't have her kill any guards.
Just her noticing the smell of smoke when she's crying in her room after the bitch stepmom just sold off all of the clothes she got from the prince for to buy more alcohol AND also demand more money.
Then when Sandra went to check on it, she found the parents asleep from alcohol, the brothers crying after they played with matches and start the fire.
She was about to step in, pause, then go "I saw nothing, know nothing, I just went out for a walk, and by the time I'm back the house is on fire....." and close the door on the kids (who mind you just sat there in shock, they didn't even try to do anything but cried for her help earlier)
Btw, she's very much aware of what she's done, and basically made this "there's no turning back now" face
https://imgur.com/a/6UQfjaM