When my son was that exact age, I came home one day and found out from my dw that he had pinched my daughter and so she had to separate them shortly before I came home. I went to talk to ds about it, and he told quite the story involving dw leaving to go get pizza and him and his sister being home alone and so he "couldn't get help" when his sister was bothering him, and so forth. My dw had absolutely not done any such thing (its actually a funny thought because my dw is a super protective mama...she's paranoid sometimes just walking out of a room our kids are in), and it was weird because when ds said it, it totally looked like he believed it...100%.
I really think there is a developmental piece here in which kids are realizing some falibility in their parents. These stories aren't an expression of how they think about us (I can't imagine my ds thinking about the very protective dw as someone who would abandon him and his sister), but an experimentation with fears larger than us. What if I am all alone? What if big people who have power over me try to hurt me?
Parenting is hard, hard work. Even with one. I have a feeling, though, that your daughter is going to turn out just fine.
I'm pro-adoption reform, but not anti-adoption.