Yeah, I'm not sure about the mother thing, my dad was the skinny one, too. Though he's fairly broad-shouldered and tall, and he's filled out more now, when he was in his 20s he was skinny as! My mother is of normal weight, also tall-ish.
I'm about 5'7" (shorter than both of them), and weigh just under 110 lbs. I've always been this way, and no matter what junk I eat, or how much I consume, I don't gain any weight. (Although normally I simply don't have room for very large portions in the first place.)
My sister (18) is Mom's build. Maybe an inch shorter than me, she's a few sizes bigger, but well within the range of normal.
And my brother (14), is a beanpole. He's in that sprouty stage, but he's as broad-shouldered and will probably be as tall and as skinny as Dad is.
So, I dunno.