who will have the twins?

On my fiancee's side, his mom has a twin sister and her grandmother has a twin.  Apparently it skips generations.  (Yes, I've heard the "generation skipping" is false but still... it's doing it.)  

Now I'm confused.  Who has the twin thing?  Is it up to the woman's egg?  Or is it made entirely from the man.  My family has absolutely no history of twins.  Only his.  So, do I have a chance of getting twins when the time is ready?  Or would his sister be the one to have them?  How does this work?  

(I know this is a fraternal thing because identical twins are technically a birth defect)