Alr so basically, Orlagh was a result from an affair that her biological dad had w/ a mistress a.k.a her biological mom.
The woman who her dad cheated on went into distress and was devastated as she was forced to take Orlagh under her care (reluctantly) and she basically hates her and doesn't acknowledge her because she looks so much like the woman that her husband cheated with 😺 so she hates Orlagh for what happened, basically taking her anger out on her.
Orglah spent her whole life knowing that she was never truly her daughter, hence explaining the, " You're not her" words.
Hope this wasn't too much and hope it makes sense 😅