My dad is a fraternal twin...but I had fraternal twins.?

basically the drs told us fraternal was carried thru the female genes. my fiances gma was a twin...but identical. I had fraternal twin boys. im just really confused because it didnt skip generations. I have two brothers and they didnt have twins. and there is no twins on my moms side of the family. just my dad and his fraternal twin. please help.