Try taking your bike to your local bike shop and have them check out your position. maybe even take a digital camera so they can take a picture of you and you can compare the photo to your image in the mirror during spin class (provided they have a mirror at your gym).
"Sit bones" is a lay-term for "ischial tuberosities" which are the lowest portion of the pelvis. The width between the ischial tuberosities determines how wide your pelvis is and is also what supports you when you sit.