Calling it: the parchment is the letter from Gerda to Elsa and Anna (from the bottle) and she says she regrets what she did to Ingrid, causing Ingrid to change.
Also, I think it’s possible that Ingrid has accidentally cursed herself with the SS curse (before coming to SB), as she seems to see negative things in her sisters that were never really there until the moment Gerda put her in the urn. She could have been experimenting with it sometime before going into the urn (unlikely), or just after Elsa released her.
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"so there’s this new show….."
"there’s lesbians in it"