They did not mention the abuse in the foster homes, she went to about 17 different ones in total, but in the movie it just showed she was dropped off at the orphanage. Mostly friends of her mother took her in, but it was just one or two of the friends dads (foster dads?) that abused her.
Because her mother was mentally ill, (but not dead) Im not sure anyone could adopt her or she didnt give permission. Marilyn did have a half sister on her mothers side. Im not clear if shes was older or younger, her name was Bereneice. I just remember reading about her.
The legend goes that Marilyn thought her dad was Clark Gable.
apparently this is common among adopted/fostered children to claim they were the love child of some famous star. I have encountered a few that have said this to me. The adopted children I know had always been adopted by extremely wealthy parents and had amazing childhoods where they wanted for nothing, never had to struggle (financially) and had a decent education. Some even had trust funds in their name.