I wouldn't ignore this. I mean, my sister is 25 years younger than me and therefore doesn't send me "helpful" emails, but I think if any of my brothers sent emails like this I would have a hard time considering them my brothers any longer.
First...aren't there physiological studies that show that a baby who is worn is actually *developing* all this little muscles needed for balance and proprioception, faster than a baby not worn, and therefore a worn baby is going to be further ahead than he or she would have been if not worn? Therefore, you are doing better for your babe by wearing him.
Third, what's wrong with doing something (figuring that she means you're American (I assume) and therefore don't need to be tying baby to you, binding him to you) that a far more ancient culture has been doing, and doing successfully? Just b/c we're American (I assume) doesn't mean our babies can *only* benefit from strollers. Babies aren't born knowing about babies r us...
And fourth, the racism. My hubby is half Korean. Though he grew up near Seattle, he was only one of a handful of Asian kids all through school (I think the immigration wave came after him LOL), and he got picked on a lot. By the time he was in high school he was in martial arts, drama, and football. He had a bit of confidence. ONe day this kid who had been egging him on for ages, calling him names like the J word your sister used, this kid just pushed him too far.
And so, in the middle of a crowded hallway during passing period, Robert YELLED out that if he was going to call him a racist name, he should get it right, and call him the word used for Korean (which funnily enough doesn't even bug Robert since it means "person" in Korean and is what they call themselves with "han" at the beginning), rather than the word used for Japanese. So now he knew and could get it right next time.
That boy wanted to sink into the floor. And he never, EVER teased Robert again.
So defininitely call her out. On all of it.