SoCal

I gather you're looking for reassurance, but you're going to get anecdotal evidence. As you would for virtually any route. Turbulence depends on exact conditions on the day, including storm patterns (if any), etc. I assume you'd be on a jet. We flew last year about this time of year PDX-LAS-BUR, with virtually no turbulence on any part of the route (and I do get nervous so I detect it). A few years ago flew ONT-LAS-BWI on a very windy day (on the ground in Ontario, at least) and don't recall much if any in-air turbulence. You may likely get fewer storms in the spring and fall, even summer, than in the winter, but in the summer you can get thermals (hot air) that can cause turbulence. We hit that flying into DFW in August (though it was just a dip). I've flown all over the world and know of few places that I can say consistently have turbulence or not. It depends on the day's conditions. No guarantees. As you can see from above experiences (some say they often hit turbulenceinto our out of LAS, while I never have).

I agree that your sister can work on overcoming her fear, but in the short run if her ultimate destination is Vegas (not beyond) and IF the fear is enough, there are options. She could drive, of course, or even take a bus (Amtrak bus from Union Station or Greyhound from a number of locations). I could quote statistics on the safety of flying vs. driving, and point out that the bus could take 7-8 hours over a boring route, but fear of flying can be difficult to overcome (e.g., why sports commentator John Madden book a bus across the country all the time). One thing you didn't say is whether your sister does fly very often at all.