No, I don't think so. At least not always. My sister was the same height as me and my brother was 6' 2". I was the youngest of three. So, I don't think it always works out like that! I'm about 5' 10". Probably shrunk some due to age though.
Are the youngest siblings usually taller? My dad was the youngest of four, and he's the tallest. My sister, who is the youngest of six, is much taller too.
