Soft YTA, you're well within your rights to refuse to sell it back to her, but given how sensitive a situation it is i feel like you should maybe be more compassionate. Lots of people are saying that she did it freely as an adult, which i guess is true, but it also sounds like one of those all too familiar stories of "i can make you a star - just do a nude photoshoot" kinda thing. She clearly regrets doing it, has gone to great lengths to recover all the other paintings, and is offering to pay you what you paid for it. All i can say is that if i were in your position I'd feel pretty horrible if i kept it, especially knowing the sadness it's causeing her.