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Amelia's life was filled with many hardships. Although the damage she did to Frank back on his childhood is by no means easily justified by anything other than her being a terrible parent in those times, it could be said that her descent to alcoholism and her insanity in those times was due to a mental breakdown that not only was caused by her husband suddenly going away.
Her entire life since childhood was rather bad.
Frank's grandmother, which was Amelia's mother, wasn't really her blood-related mother…
Indeed, Amelia was an adopted girl.
Since she had memory of her own self that she was always alone.
When she woke up in this world, her mother was not here, nor her father.
She was raised by nurses and then thrown into an orphanage when she learned how to walk.
Although she went from hand to hand while being little, as she had many caretakers, she never truly connected with anyone, and was always alone.
Check "The Protagnist's Sister Is Actually the Strongest"!