You can keep quiet without being complicit and without being a gossip either.
This is the bit I see differently.
The problem is that Sarah has made the OP complicit just by telling her. OK maybe she's not joining in with the lying and arranging weekend breaks in her parents' empty holiday cottage for them, but still. She is complicit if she knows and doesn't say.
Let's skip ahead in this story. It comes out. John is now ready to go, having got his ducks lined up, accounts/money etc. protected, spoken to his solicitor, they've rented a house, whatever. They choose when best for the axe to drop, Emma and the kids are told, lives blown up.
Now it's everyone's gossip. OMG. OP, can you honestly believe Sarah could have done this? Etc etc. Including very likely Emma herself turning to OP for support. OP's husband being astonished. etc.
And in all of that - what does OP have to do? She has to lie. Lie and lie and lie. Or, tell them she knew and face the consequent loss of trust in her forever. By her OWN HUSBAND.
And why would she do this, and risk all this? With an almost 100% chance of damage to her own life?
Ah yes. Because Sarah wants to keep fucking John on the quiet for now, while they sort out the practicalities of getting together.
If OP really wants to do something, she should:
Tell Sarah that this is bollocks and she hates having been put in this position.
That ok fine it's cool you guys are in love, etc, but the humane focus now must be on how to minimise the hurt to Emma and the kids.
Tell Sarah that she needs to talk to John, and John needs to split up with Emma. Not citing Sarah as the reason.
Then when that's all settled, they can 'start' seeing each other.
That way, Emma never has to know about the depth of deception and lying. She won't have to re-write and effectively lose four years of her married life and her female friendship.
Basically, Emma will presumably be devastated, but she won't know the full truth.
And when she's ok, and the kids, then Sarah and John get together. And then it's just Sarah and John who are bad and hurtful and Sarah gets dropped from the circle, but at least Emma doesn't get so totally fucked over.
If you really want to make this happen, tell Sarah either she gets her shit together and does this, or you tell your husband and Emma.
BUT the above is pretty active and isn't at all what I'd do. I would just talk to my husband. OP if you keep this to yourself, you'll end up somehow taking the rap for it.
And. Big And: you have zero guarantee that if you keep quiet, when it all comes out Sarah won't say 'oh but OP knew about it all along'. ...