Your cousin definitely has cause to be worried about the true circumstances surrounding the removal of her adopted children from their birth mother.
These Social Workers constantly lie about the background of the children.
You have probably already read this post that I made, where the claim was that the mother threw the baby out the window -- RIDICULOUS! And then the baby was adopted to a woman who was told that the mother never ever cared about her baby.

https://steemit.com/familyprotection/@canadian-coconut/when-kidnapped-children-get-adopted-familyprotection-series-tammy-s-story

There will often be the slightest bit of truth to the story, which Social Services embellishes. My best guess would be something like this:
-- The 2 youngest children were babies/toddlers, who still slept soundly though the night (like mine did) and the mother shared a bedroom with them. Perhaps the boyfriend stayed the night in mom's bed, and while the children were soundly asleep in their cribs, they quietly had intercourse. None of the children, especially the older children would have ever seen this.
Probably the Social Workers hounded the mother with question after question, like "does your boyfriend ever stay overnight?" "Have you ever had sex in your house?" "When/where did you have sex?" "Where were the children?"
And the mother would have incriminated herself, not imagining that they would remove her children over that.

Now I am not saying that it is wise for a mother to behave like this, but what I am saying is that they surely blew the whole thing completely out of context when they relayed the story to the adoptive parents.

Thank-you for sharing your story and your cousin's story here.