In the beginning, I thought she may have forgotten about the girls being in the car because of the argument, but that's obviously not the case if the fight happened inside. Now I've been wondering how much she relied on the older daughter to help care for the 2 little ones. Is it possible that the fighting woke up the girls and the oldest one picked up the baby to comfort her? When both parents went out the door, the older one may have followed because she was confused about what was going on, and the 2 year old followed her out. If the mother did take off to follow the father, the girl could have run to try to catch her and tripped, dropping the baby. Of course, that doesn't explain what happened to the 2 year old.
Another possibility is that she did load up the girls to follow him, but changed her mind because he was already out of sight and she wasn't sure where he was going. If she was under the influence of alcohol or something else, she may have just gone inside, gone to her room, slammed the door shut and passed out, expecting the daughter to bring in the little ones. If the girl wasn't told to do it, and was tired and/or scared, she may have left them in the car thinking her mother would go back out and get them. I have known a few women who would fly off the handle like that, and the kids learned early not to ask questions until mom calmed down. I really do want to believe she didn't do anything to purposely cause their deaths because we've seen too much of that already. MOO