Welcome Graced333:
Congrats on achieving sobriety! I'm so sorry you are in such a tough situation. You mention the accusation occurred 20 years ago. Is it her step-father that your daughter accused? Is everyone still in contact with each other, or is anyone estranged?
Has your older daughter been diagnosed with BPD? How old was she when she made the accusation?
If you are able to share more details, it could lead to obtaining more helpful information for your situation.