KingJoffrey I think that, with support, you are going to have to face this at some time. If you still want contact with your MIL and not with SIL but they actualy live together, then at some point your SIL is really going to twig and is going to kick off.
From all you're said you are absolutely making the right decision to go no-contact btw. If you looke back in 5 years' time after NC and think again that you were nearly suicidal over her aggression, then I think you'll see very clearly that she is no good in your family's lives and you will wonder why you put up with it for so long (just wondering - have you got a habit of putting up with people being unreasonable and unpleasant? she siunds awful and certain she's no good for you).
Ok, how to manage the fallout? Someone said above that repeating "no im not having this conversation" is a good idea. I second that.
I would actually think the best way to handle this is to be aware you're coming up for a fight, to think of your husband and family and draw strength from that. And to -answer- the phone next time, to tell her very clearly you want no more contact and that that is that. Do not get engaged any furhter at all. Think of her as a squalking parrot if she says anything at all, and throw a blanket over the cage- do not listen and end the call.
In all further communications that SHE begins, keep repeating "no I'm not having this conversatoin" and walk away / put phone down.
It will mean facing the war for a time. It will, and that will be hard. But this is for your family and your husband and yourself. Knowing what's coming will help you deal with it.
Ensure you do things to look after yourself. Exercise, mabe a massage or things you like. And if you can, counselling with a skilled counsellor you click with. Keep talking on here - you'll get bagloads of support.
Also, ask your husband to TELL her to stop contacting you all too. Im sorry but saying 'i dont know' to him if he offers to tell her to bog off, is not helping you.
If necessary, change your phone too and let the MIL contact you only on the old phone.
It may be that your MIL cannot handle all this and you end up accident;y NC with her too, if SIL makes her life too hard. But this is a small price to pay when you have been feeling suicidal over it. This is pretty appalling, you know, that it's got that far. Your health has to come first!
It'\s terrifying facing an awful woman like this, but you have help and support and again - your health comes first. You gotta, in this case, but yourself first