Hi, thank you for your question. Given that you were not aware of ypur paternity and that you had been informed she was your stepdaughter, as well as there being no formal child maintenance application against you, you would not be liable for any "unpaid" maintenance and her threats will not be fruitful as she has no claim against you - in any event if there were child maintenance arrears they would be claimable by the mother.
However, depending on her age and if she is at university she may be entitled to pursue an application for maintenance if she is unable to support herself in university.
I hope this assists you. If you found this information helpful please provide a positive rating using the stars at the top of this page. I will not be credited for your question without a positive rating. Thank you