I sympathise - a situation like this is a nasty one to get involved with and you want the best but aren't sure what that is.
It's hard when one person in a partnership wants to be poly and the other is faced with it. Sometimes you can read all the books, and read all the internet and think you can deal with it, but when it comes down to it, you realise that it just isn't for you. I think that may have been what S has been going through.
In other words she has learned something about herself.
Now N and S have non-negotiable bottom-lines that conflict with each other. They need to get that sorted out. It doesn't matter what triggered it, or what family and friends think. You were just a catalyst - if it wasn't you it would have been someone else.
Your responsibility, as I see it, is to allow them to get themselves sorted out, now that they have this new knowledge. You can't help and, given your closeness to the situation, it would possibly make things worse.
Since S wants things to be monogamous, I would be distancing myself from them for a while - give them the space without her having to worry about what you and N are doing together behind her back. I wouldn't surprised if a lot of monogamous conditioning starts kicking in and you don't want to be close to that, unless you feel comfortable in a role as a punching bag.
I wish you luck in a difficult situation.
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