Abuse that long-lasting and extensive had surely caused some pretty severe mental issues, if anything it's unreasonable to assume that the teen was capable of making rational decisions here.

Perhaps you read some more information than what was included in the article? Apart from impregnating the teen at the age of 16-17, I did not see any further information about abuse. If you are suggesting that it is unreasonable to assume someone who is physically or sexually abused by their step-father is incapable of making rational decisions, you have seriously insulted the millions of people who live perfectly healthy and "rational" lives after suffering childhood abuse. It is incredibly ignorant to assume that because someone has been abused that they most likely are mentally impaired and incapable of rational thought.

She's not even an adult yet

Actually, in most parts of the world and much of the United States she is. She is only months away from holding all of the legal responsibility of every other adult in America.

Maybe she even thinks it's not unusual to live that way.

She thinks it's not unusual to live with children starving to death in your presence?

can you really expect someone who was raised in that kind of life to make informed, rational decisions?

Yes I certainly can. Millions of people have suffered through neglect/abuse/rape/ and other trauma and are capable of not letting a child starve to death when their are accessible resources all around them.

As I have said in several comments now, if she is found to be developmentally disabled or have a psychological disorder that renders her incapable of rational thought, she should not be held responsible. But simply being 17 and having been raped or abused does not automatically absolve you of responsibility when you are the only thing between life and death for a 3 year old.