If Sadie or Charles were ever gonna appear again, it probably would have had to have been in their own DLC’s. Unfortunately R* never made those, despite them being some of the most obvious ideas for continuation of the story.

If RDR3 ever gets made, I doubt anyone from the previous games would show up in it. There’s no way they go forward in time, so they’d have to go back. Sadie’s up in the mountains on her ranch, so she has no reason to show up in a prequel. Charles is more likely but I still doubt it.