Sounds like you've already made up your mind.
If everything you've said is true (not questioning YOU, but assuming the stepdaughter is truthful about the sons not being around the last 25 years) then you should feel no remorse or obligation to sell them back. Long-lost family members surface like mushrooms in shit when parents start passing away.

My aunt was estranged from her husband for 20-some years. For whatever reason, they never actually divorced - he lived out his days with another woman, and she moved in with her sister. He passed away a few years ago. He had never changed his will, so his assets went to his legal wife, my aunt. We're talking roughly $80,000 - not life changing, but certainly considerable. Well, in his will, he asked that $10,000 go to his 2 daughters from his previous marriage. I didn't even know he HAD 2 daughters from a previous marriage. My aunt hadn't heard from either of these women in 2 decades, and all of a sudden she's getting these big, smiling phone calls almost daily, "Hi Auntie! How are you?? Have you settled the will yet? No? Ok, well let us know if you need anything! LOVE you, Auntie!"
So, she did as her husband wished - she sent the money, along with a scathing letter about their disappearance 20 years ago and their bright, shiny, magical RE-appearance.

She hasn't heard from them since.