No illogical reasoning on my part. Just follow the actions. She waited until the coast was clear--had she not known what she was doing was wrong, she might have asked for help--"Gee, I'm really swamped this morning. Gotta kill the kids and I have only an hour to do it. Want to help?" She did not do that. She waited, killed them, called the cops, then called her husband. This is premeditated murder--witness the actions--and then she turned herself in. The husband didn't arrive home until after the police were there, something I'll bet that Andrea counted on.
Mentally ill can mean a variety of things--not necessarily psychotic. Depressed people can be described as mentally ill. The thing is that she knew enough to take precautions to insure that she got the killings done and only then did she call in the authorities and finally her husband.
In my book, that is conscious reasoning. She knew.