I'm the oldest with 2 sisters, but it never occur to me to judge how people behave by being the only child/eldest/youngest at home... hmmm...

People who look at stereotypes are ignorant and they refuse to look at you for who you are but look at what you are. To them, someone who come from a bad family will always turn out bad and those who come from good family will always be good. This we know is never the case.

I suggest you ignore what they think about you and prove them wrong, that will teach them a thing or two.