One thing I would say about looking at your relatives is that you also have to consider their lifestyles. I have relatives who are quite a bit larger than me but they also don't live the same lifestyle. The minority of my relatives who do live similar lifestyles look quite a bit different from the majority of my relatives who don't.
exactly!! you can use your relatives with different lifestyles (and similar genetics) to see what might happen if you slack off in your training and take up the 'fast food' habits of many people… I have a set of seriously LARGE people in my family who enjoy eating and never exercise, keeps my motivation up there everytime I see them… as you age and you metabolism slows you have to be a bit more vigilant, well I do at any rate..