Count me in as one who doesn't see the fb status as any of my business.
I really try to keep out of other people's relationships beyond offering a shoulder for a friend to cry on during a break up.
All fb tells me is that the person concerned is communicating that their marriage is over and a new significant relationship has started. Better it's honest and out in the open, if so. If I disapproved strongly about what someone was doing, I wouldn't stay fb friends.=
Consider this - when my stepbrother and his wife had first split, he was still hoping to reconcile with her. She is also the godmother to my niece. We all found out that she was dating someone new through facebook (it was something like 3-5 months after they separated). It was not a very nice way to find out that she clearly didn't intend to stay in the marriage. Thankfully my niece is too young for facebook but if she had been, that is how she'd have learned that her godmother was not going to be part of the family any more.
She defriended me (and the rest of her ex-in laws) shortly after this which makes me suspect even more strongly that it was a statement, a kind of shot across the bow. Tacky, classless, and hurtful in my book (though it did make me think my stepbrother will be better off without her in the long run!).
I think there are some separate issues happening here. First, your stepbrother hoping to reunite and his XW starting a new relationship
is something that they, as a couple, needed to work out. One of the reasons I don't like 'dating
while separating' is because if I were in a situation where I was either dating
a man that wasn't yet divorced, or was the person whose spouse was dating
someone else, I wouldn't feel very respected. Since I personally believe that committed relationships
should happen only between two people, this is not something I would be ok doing myself and wouldn't feel comfortable knowing others are doing it.
That being said, what occurs between consenting adults is their own business and it's not my place to impose my views of what a healthy relationship
looks like on them. No matter how strongly I feel about it. So, your stepbrother wanted to reconcile and I can see, as a family member, how hurtful it would be to watch him get his heart broken. And I can even see where that would sour your impression of your SIL. But that doesn't mean that she's obligated to keep her feelings secret. Clearly, she didn't want to reconcile and while I do think she should have said something to her husband and maybe your niece's parents since she's the godmother to their child, first, I do not agree that she was wrong to post her relationship
status, nor do I think she owed an explanation to anyone not directly affected by her actions.
As for your niece, I do not agree that her ending her marriage automatically means that she is no longer part of her life. Her relationship
with your niece is not and should not be tied to her relationship
with your stepbrother. Now, your niece's parents do have a say in how much they do or don't want her in their life based on their perception of her lifestyle, but that choice is theirs regardless of what she did or didn't post on FB. The FB posting itself, has nothing to do with it.