I don't quite understand what you mean. I did choose that option that says nell is the daughter of fairy queen, but afterwards it asks me to find a contradiction, which is the second fairy queen clue but I don't have it so I can't choose anything.
Here:
Visit stables
Talk to Nell
Invade mind
Nell's Origin (dark secret)
Nell is the Fairy Queen's daughter
"... she was told the truth."
The Fairy Queen insists that she had no children with human
"... she told the truth. She has had no children with a human."
Nell is the Fairy Queen's daughter
They are two entirely different fairy queens.
"You are the current Fairy Queen's sister."
Remove Inhibitions
End spell
Convince Nell to come to the Fairy Queen with you
Threaten to breakup with her if she doesn't go.