I think Shelly plays the game "heads I win; tails you lose".
I no longer have any patience for that, and I would be all 'broken record' if I were in Bob/Betty's shoes: "But it was OK when you didn't invite us, wasn't it? How is this different? No, seriously, explain how this is different". Lather, rinse, repeat.
{{Hugs}} to Bob & Betty, and a (somewhat sharp) "Grow up!" to Shelly.
Oh, I am so lucky I don't know these people; the urge to
say something is so strong [probably echoes of my own circle's dynamics pushing some of my buttons?
]; my low/no-contact 'friend' MaryMary is great at the "If I do it, it's OK; if you do it, it's
wrong" game, and I am
so not missing our interactions. A mutual friend asked if there was a problem [later turned out Alice is not speaking with MaryMary either], and I said "I'm just having a holiday from having my head bitten off, being criticised and pulled to pieces, and being judged (as being in the wrong) for every.last.thing."...
Turns out "holidays" are catching! I hope Bob & Betty manage to let Shelly finish out her sulk in peace without feeling any need to step in and make things 'right'.