Who knows, you’re throwing out a lot of assumptions, maybe she wasn’t showing any kind of suspicious behavior before all of this unfolded. There are a lot of questions to be asked about this and there won’t be many answers, nobody knows what happened inside that house except for her, if she did suffer from PPD, she could have hid the symptoms until she snapped, you don’t know how this family lived their lives. The “system” and other family members shouldn’t be blamed for this until more information is available.