Only paternal twins run in family's and only on the mother's side because it has to do with how and when the eggs are released from the ovaries. For example, If your mother or maternal grandmother had paternal twins it's very possible that you will have paternal twins also. Identical twins do not run in families. You shouldn't worry about it because it seems all the twins are on your fiance's side of the family and his gene pool has nothing to do with you're ovaries... lol.