Well, I'm a twin, so I bet that counts! My non-identical twin sister and I learned to say the word "fraternal" when we were three, because someone told us that's what we were. Big word for three-year olds! When we got to be about 10 or so, we decided that we are sororital twins, because fraternities were for boys, but sororities were for girls, so sororital should be a word, right? Can't beat that logic!
Do twins run in our family? Yes, yes they do. Our first cousin has a set of twins, a boy and a girl. Our shared Swedish 2nd great grandmother, Ottilia Boquist, had a twin sister. Her husband's father, Johan Magnusson, also had a twin sister. So the twin gene appears to be from that Swedish line.