Great art connects. There is a reason most coming of age films take place during those formative years in high school, and often involve a romance fighting for narrative space against parental issues. The reason is that it is so universal, so non-specific, so Coldplay, that everyone can kinda sorta relate to its nothingness.
Little Sister is highly precise. A former goth teen turned nun returns home after an email from her mum informs her that her brother is back. Where he is back from is slowly unraveled, as is why Colleen left her misfit angst behind for a life subservient to God.
By showing what happens once she has already come of age and found her calling, Little Sister nails the point that we never actually come of age. Her mother is a neurotic wreck with a drug reliance and unacknowledged mental health issues, and she looked for meaning in all the wrong places. Her brother is a changed man, and the challenge is in finding her sibling within what he considers his new normal.
Why it all works so well is that it has that amazing true-to-life feeling that everything is just slightly off. Colleen has been away for three years, so while her family consists of the same people, slight changes have made them all the less familiar to her. Those shared memories and core relationships remain, but each individual life has gone on living while the other has been absent. The terrifying coming of age prospect is the loneliness in such a thought.
A particularly moving scene involves a freshly hair-dyed Colleen lip-synching to industrial metal while smearing fake blood on a baby toy. It is so not universal and yet the authenticity and heart behind its motivation is why it, and the film as a whole, is ultimately incredibly touching. Colleen and her family have lived a believable life, and continue to do so, both together and apart. Free from tropes and traditions, Little Sister is also free to connect with anyone who has no idea what they are doing and is just trying to be happy.