I agree with the original poster. I can eat all day everyday, and it doesn't matter how much or what it is I can't gain weight to save my life.
I find it VERY hard to believe your brother nears anywhere close to 10,000 calories a day! and stay slim with ZERO exercise
Most professional bodybuilders don't even eat 10,000 calories a day and they have the highest metabolism of anyone
I can eat fast food, drink beer, and sit on my skinny ass all day, everyday, and you won't see me put on more than a few pounds. Believe me I have been on high calorie diets and it did nothing for me.
Everyone has different types of bodies. A body builder may eat a high calorie diet and gain tons of body mass, but if I do try it won't happen at all. Believe me when I was younger I got tied of the tall, long, skinny jokes and tried to put on weight and bulk up but it just wouldn't happen.