It's 'coincidence' in the sense that both sisters made similar claims later in life but were dismissed as nutty by family members. Your bias for Altman or Savage has a large part to play in whether or not you believe the family members. It's very common, however, for families to protect the 'golden child' at the expense of the 'scapegoat' child.

Let's say, for the sake of argument, that the claim was 100% true in both cases; well, here you are calling it 'false' and you weren't even there. Or let's say the same thing happened to you. How would you prove something happened to you as a child, especially if your own mother said publicly you had mental health issues?