She asked me for a harmonica, so I went into the hills and killed this guy (and his chickens, and his pigs, and his horse, because they knew too much) for it and when I came back with one and looked for her, she was… gone. Was is something I did? Kind of.
It’s one of them vexing bugs, Rockstar has confirmed, and it’s also why you’re missing John Marston and his family. You didn’t realise? Me neither, but now that I think about it, there’s been much less awkward squabbling in front of everybody.
“If you completed the mission ‘Polite Society, Valentine Style’ at the start of Chapter 2, but needed to use the Retry Checkpoint feature, then Sadie, Jack, John, and Abigail will only appear in camp when they have missions for you to complete. We are aware of the issue and are currently working on a fix to be included in an upcoming Title Update,” the studio explains in its support FAQ.
“In the interim, in order to experience all vignettes and conversations, players must complete the ‘Polite Society, Valentine Style’ mission without using Retry Checkpoint. If the player fails any portion of the mission they should abandon and restart mission, or load a save before the mission failed.”
If you decide that you’d prefer some peace and quiet for now, you can wait until chapter four when the Van Der Linde gang is reconvened because, uh, reasons.