Hi Amanda!

I'm not Emily Post, but I don't think you'll run into any etiquette problems wearing your sports bra & cycling shorts. However, please keep in mind that cycling on the road is different than cycling in your spinning class. For one thing, when you cycle on the road, you will not feel as sweaty because as you move through the air, the sweat will evaporate. Also, you're out in the elements, so you will be exposed to the sun.

An added benefit of a cycling jersey is you usually have pockets in the back to store things. If you store a route sheet or anything made of paper in the back pocket, put it in a plastic baggy, or you'll have a very soggy piece of paper by the middle of your ride. (Ask me how I know!)

It sounds like your sister is looking a the baggy mountain bike shorts. Here's a link to the mountain biking shorts offered by the Team Estrogen site:
Mountain Biking Shorts

Hope this helps!

-- Melissa