I don't think so. A mother known as loving. No motive to kill the son. It would have looked like an accident and she was only able to save one. I doubt the prosecutor would have even done a thorough investigation or try to press charges Plus if I recall the family is wealthy and a decent lawyer would definitely be able to get such a case thrown out for lack of evidence.
And usually when someone is obviously murdered there's forensics and such stuff. So, sure, two people could kill a third and back each other up, but in most cases there'd be forensic evidence to tell the true story And the mom didn't do anything, except try to prevent them from falling What possible proof could there be to the contrary?
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"No! He is imprinted on you like a gay duckling. If you don't wean him off you slowly, he'll die."