I don't think I would tell your W. She already knows the deal, and she's pissed you didn't see it before now.
It's sort of like how you don't tell you W every thought you have b/c some would just be too hurtful.
Your call, but I would not tell her the details. What, really, is the upside in telling her?
I would just tell her you understand now what she felt way back when she knew something was up with your mother. And that, again, you're sorry for not seeing it sooner. But, now you understand and do not approve of how W's been treated. No reason to go into details.
If she point blank asks if your sibs have told you anything, then you have to be honest with her, but I wouldn't bring it up.
