It really rather depends on you.
It could be there is a misunderstanding that has got blown out of proportion somewhere along the line. Perhaps Sarah wonders if you’ll blame her for encouraging you to leave your husband? I suppose the only way to know for sure is to be firm you want to talk, I would simply say you feel like you’re being deliberately ignored and excluded and feel hurt. I would just say that you value the friendship and hope that whatever the problem is it can be resolved - but if they do not want to be friends anymore to please just be honest and say so.
And no, I do not think it is unreasonable to expect some confidentiality from a friend that you’ve told personal stuff to. And gosh no, this isn’t what friends are suppose to do to each other at all. But, I would also caution from immediately assuming that these two have just been gossiping behind your back - they could perhaps have been discussion how best to support you - granted their behaviour doesn’t really support this - but having seen a friend being hurt terribly by misunderstandings, recently, I would suggest just having a chat and asking what on earth is going on before you assume these two are just not very nice people.
Just to note: I find though that when personal circumstances change and you change from being the one offering support to needing it…... well, you tend to find out who your real friends are then.
Joxy.