I'm so scared :/

I've recently found out I'm pregnant, I have identical twin girls already (age 6) as much as I love them to bits I'm so scared incase I have twins again, I know ID twins don't run in families but my dads sister had feternal twins. I am unsure if anyone else before my Aunty had twins on my dads side of the family :( so unsure if I carry the gene or not !! so nervous !