While “dirty little secrets” could refer to a wide variety of scandalous doings, it is more of a literal translation here.
Lucy’s mother is a compulsive horder.
So far, Lucy has been able to keep this “dirty little secret” by keeping her home life and her social life completely separate. She NEVER lets friends come over to her house because she is sure that they would be so disgusted by the inside of her house that they would never talk to her again. Lucy finds it extremely hard to live in a home where there is barely enough space for a walkway between garbage piles, but that is nothing compared to the stress of living with a mother whose moods are as unstable as the stacks of magazines lining the hallway. Lucy’s mom often accuses her of losing things and cannot admit that her own hoarding is likely the source of the problem, so Lucy does the best she can to placate her mom and/or to stay out of the way.
As far as the outside world is concerned, Lucy is a perfectly normal teenager who lives a perfectly normal life. In reality, though, Lucy is so ashamed of her mother’s mess that she can’t even bring herself to call 911 when she finds her mother’s cold, dead body under an avalanche of trash. Shocking and heart-breaking in equal measure, this story does an amazing job of shedding light on the impact this mental illness has on both the hoarder and his/her family members. And, while the surprise ending wrapped things up rather swiftly, it was definitely a believable conclusion.