His sister is Junior's mother, who either killed herself, as I think we were given to understand last season, or faked her death (there was a funeral, at least) and moved to whatever city she's now painting in. (The scrapbook he was looking at was Pauline Verdreaux's, and Sherry Stringfield is credited on IMDb as Pauline.) That's not to say he didn't have ANOTHER sister, but there's already plenty of mystery around the one we know about. I think we're supposed to think she has some kind of precognition about Dome-events, so she could have predicted Drownsie years ago when she drew that sketch.
Or...maybe Drownsie is Pauline, as she looked when she was a teenager. Literally anything is possible on this idiotic show.