What is your advice to a girl who is suffering from pangs of conscience due to her imitation of sexual movements with her cousin in childhood, and how can she overcome her fear of her secret being exposed?

There is no need to worry simply out of fear that the paternal cousin might expose her matter, because these anxieties could be from Satan. It is unlikely that the paternal cousin would do that, as she might expose herself as well. It is advised to turn away from these thoughts and seek refuge with Allah from the accursed Satan when they arise, and to increase in the remembrance of Allah and supplication, and to ask Allah for concealment in this world and the Hereafter. If the questioner was accountable at the time of committing the forbidden act, then she must repent to Allah the Almighty.