Jake Scolecite wrote in his report that, apparently, the sperm donor of the babies was supposed to be Ethan Diamond himself.
Ethan and Celine were in an incestuous relationship, but they didn't want to risk inbreeding while still wanting to get pregnant.
Celine, six years ago, was still healthy enough to have a baby herself with her own eggs, but to make sure their babies were healthy without any disability, she decided to search for an egg donor.
That way, she could bear her brother's babies in her own womb.
But it was later revealed that Ethan Diamond used another man's sperm for an unknown reason.
When the children were born, they looked nothing like Ethan, who was supposed to be their sperm donor. That was the start of Celine's suspicions.
As they grew, they looked and behaved even more strangely, making Celine hate them more and more. That was when the abuse started.
It wasn't always visible. Celine would also hit them, but more so, they were emotionally abused.