I started a group like this in Massachusetts and it's still growing like crazy after five years. It might be more energy than your sister cares to invest but why not start her own club? If there's a certain speed and distance she likes, there are probably tons of other people in her area looking for the same thing.

A simple way to get started is to find two or three others who could become "regulars". She could do this through a meetup group or Craigslist or just with some friends or co-workers. The secret is to keep a very regular schedule and find safe, interesting routes. Start off on the easy side with rides that are maybe only 15 miles long. An average speed around 10 mph is probably the most inclusive way to start. She might even make the first few rides on a local bike path.

Collect the email addresses of everyone who rides and send an email blast a couple of days before each ride describing the starting location and time, total distance and maybe total elevation gain. Use something like RideWithGPS and attach a cue sheet to the email that riders can print at home.

The first year, there might be only 2 or 3 riders per ride. With consistency, future years could see 5 then 10 then 20 and so on.

If a club that matches her needs already exists, great. If not, a little effort goes a long way towards filling the void. Good luck!