Twins hereditary.........??

Both of my grandfathers are twins, and no one else in my family have had twins. Who is the carrier of the twin "gene"? I always wanted twins when I was younger because I thought it would be so fun, however, now that I'm older and trying to conceive, well...It makes me nervous that I may have twins due to my family. Is this something that could happen?? Does it really run in the family?? Thanks :)