it has to do with a lot of things. less weight is obviously going to make a difference, but that doesn't mean that just because someone's lighter that they're not going to have the same problem, because they can also be doing something wrong that could cause the same problem. for example, if you're pedal stroke isn't smooth, than you're going to move around on the saddle and that could cause soreness. also, i'm guessing you're pretty upright on the saddle, and believe it or not putting your weight on the rear part of the sitbones is going to cause soreness. and the opposite holds true too, getting down on the hoods or in the drops (translation: aerodynamic) and putting your weight on the front part of your sitbones is more comfortable and will cause less discomfort. there could be a LOT of reasons why it happens to you and it's not happening to your wife and your daughter, but unfortunately that's how cycling is, every pedal stroke your body makes thousands of movement and muscle changes and if any of them are off you will pay the price...

and as far as the sit bone structure thing goes, having the right size saddle is very important, but that doesn't necessarily mean that all of your pain is going to go away if you find it. sure, it's a step in the right direction, but also have someone watch your pedal stroke and watch you ride to see if they notice anything of the ordinary...