A minor family squabble

I come from a relatively large family; I'm the second oldest of seven. My sister Susan arrived a little more than a year after me. Consequently, she was just a grade behind me all through school.
One day a teacher asked me, as Susan and I stood in the hall near his door, "Cosgriff, just how many of you are there?" I answered that I had four brothers and two sisters.
"No you don't," Susan corrected me. "You have five brothers and one sister."
I was caught off guard for a moment before her confusion dawned on me. "Nooo, YOU have five brothers and one sister. I have four brothers and two sisters," I explained, surely in that denegrating voice elder siblings speak with towards their younger ones caught in obvious mistakes.
But I'm right, aren't I?