Evangelista your post is very moving. To me and this is again my opinion is that your sister told someone about her hearing God tell her. Andrea Yates did not tell anyone what she was or was not hearing. She planned out how she would do this. She said herself she waited until there was no one around her.
Your family took matters into their own hands to protect this child. I applaud them. Someone took the time to intervene. No one had that chance with the Yates children. The dad could have paid more attention to his wife's actions. Again there is a fine line between sanity and insanity as well. None of us will ever know for sure if Andrea was sane or insane. But again I feel that a person that is truely insane will not plan it out like that..temporary insanity is like a snap. it happens so fast I think. I am no expert though I will admit that to anyone.
I can feel your pain but you did the right thing in your case. More people should get involved in those kinds of issues.