@ jimmyclear
I think you misunderstood. She doesn't know that her aunt killed her mother. She knows her mother died (and thinks she died a hero, I believe). When her mother died, as the daughter, she should have inherited her mother's "Garm", and with it, memories of her mother that are stored in them. Her aunt refused to let her have the "Garm", though, because the aunt would rather be hated than for the Selka to know that her mother ended up going against humanity by seeing the memories.