I have 2 sisters and we've lost, gained, had kids, lost gained etc. over the years. but it's weird, my youngest sister no matter what, always looks the best. She's solid as a rock. Then it's me, then my middle sis.
I think, part of it is the amount of physical activity, your diet and genetics.
Youngest sis and I are much more physically active with jobs and such than the middle sis, so middle sis seems more fluffy.
Anyway, on a funny note about body types,

a few years ago, I stopped in a quick trip about 40 miles from home for some gas, went in to pay and get a cold drink, I was bent down, grabbing my soda out of the cooler, my youngest sis worked in this place, but wasn't there that morning, a salesman, popped in, and unknowingly, started to give me his usual ration of BS that he normally gave my sis. I knew this guy, he was so embarassed when I stood up and turned around and said "hey, that's hilarious, but I'm not Lex!
You could have heard a pin drop!
Long story short, our butts look so much alike, he had no clue.