I'm great with kids but intermediate family matters are different. My little cousins love me as if I was their older sister. I can take care of two 10 and 8 year old girls and then my 4 year old twin boys easily. But i can't seem to deal with a measly 11 year old boy.
I guess it depends on the age difference between the two, really.
My brother used to really bother me. We're six years apart. We get along okay, now that he's eight... Did that make any sense whatsoever? The only thing that bothers me is that he has an annoying friend across the street. He tends to be a jerk to my little brother, but he just lets his friend control him. His friend likes to scream at people with a tone as subtle as that of a velociraptor's.