My opinion: she’s a psychopath. She had deep resentment toward one or both of the parents. (Was she ever romantically involved with either parent? Or did she want to be? Feelings of betrayal here.)
She planned on hurting the babies or at least one of them to get to the parents. I think she planned to make it look as though the injuries were caused by the parents, which likely means she did whatever she did right after they left. (To make the timeline murky when the medical examiner/doctors tried to create a timeline of the injuries.)
I’m wondering if the deceased twin was actually unconscious or already deceased when the parents returned, but she distracted them with the other baby’s injuries and they left for the hospital without realizing the other baby had been injured too.
I can’t explain why but I have this feeling that she may have bitten the baby’s penis. That would cause redness, swelling, bruising, and possibly abrasions from teeth.
Her goal was to cause great suffering.