I'm looking for a road bike for a new heavy rider

I suggest that you post on the Bike Buying Advice forum, if you haven't done so. Also look up posts by Sierra Slim on that forum and other forums here. She was/is in a similar position and lots of us here have given her advice and encouragement. She has purchased a bike and I'm sure that she would be happy to share her experiences. Is your sister able to ride now? What has she tried riding, and what issues, if any does she have (bending down over drop bars, getting her leg over the saddle, etc.)? The obvious initial advice would be to take her to a good bike shop and have her test-ride some bikes. But don't be in too much of a hurry to buy. And forget about surprising her with the bike. She needs to ride before she buys. Steve

Thanks Steve, good advice. Yes, she can ride a bike now but it's more of a grandma style bike. I have an LBS that I love to work with and they are great but I'm just worried about a standard road bikes ability to handle her weight.

+1 on Steve's advice. There are come considerations to keep in mind though. With your sister's weight issues, you first should get her fitted. Fit is the most important point for any bicycle purchase, but it is even more important for people who have certain issues that may preclude some of the more common adjustments and tweaks to compensate for a frame that is a couple of centimeters bigger or smaller than optimum. A proper bike fitting will take 2 - 3 hours and it is not a simple procedure. It also costs close to $100, but often the bike shop will greatly discount it or throw it in free if you purchase the bike from them.

Most aluminum frames are sturdy enough to easily handle 300 lbs. For CF, however, that would be pushing it and the ride quality would probably be too flexible to be comfortable. Also, with that kind of weight, your sister would probably want to start out with touring bike wheels. They are a little wider than sport road tires and have a higher spoke count, usually around 36 - 40 spokes per wheel. They are also heavier. As the weight comes off, she can easily change out the wheels to a lighter wheel that uses a thinner sport tire and has fewer spokes.