SP told the police that Grace had an "emotional disorder" and "behavioral issues"; I figured that was why she was labeled as having a disability. She did get disability payments for her, and the amount is what you'd expect for a disabled child. Also, the school noticed that her letter was too advanced for her skills--a typical 14-year-old is nearly at adult level in writing, so Grace might have been behind.
My working theory is this: Grace, because she was adopted, and because she was mistreated, had some kind of emotional or learning disability--the sort that children get when they aren't loved and don't get a safe environment to grow up in; the PTSD and attachment issues from abuse, the emotional instability from dealing with an erratic, abusive parent.
She may have had some physical issues--that medical implant suggests she did. But it could have been anything--an ear tube, a shunt, screws and plates to fuse broken bones or straighten a crooked spine; a pacemaker or automatic insulin pump; a cochlear implant. There are so many possibilities, and not all of them imply a disability.
If SP actually thought that Grace was disabled, she may have used it as a way to rationalize to herself that Grace was disposable, not really going to amount to anything. On the other hand, she might have used the label of "disabled" just to get that monthly payment, and believed that Grace was a rebellious, badly-behaved teenager who needed to be kept in line.
One thing I do know: If Grace was rebellious, then it was because she was fighting to survive.
