Quentin was the Jennings' great-grandfather because Chris said that Quentin's daughter Lenore was his grandmother. And, yes, Quentin knew he was related. In fact, one of my pet peeves is that, except for one extremely brief reference to it in Ep #962, nothing more was ever really made of that fact between Quentin and Chris - or that Quentin was responsible for Chris' curse.
The reason I think that perhaps Chris, Sabrina and Amy's departure got so little fanfare was that they were still hoping at the time that one day Don Briscoe would return. There were actually plans for him to play Randall Drew in 1840, but it just didn't work out.