I carry the Twin Gene.
One of my sisters had two children a few years apart.
One was a boy the second was a girl.
The first born, the boy, grew up and married and his wife had twins.
It was not conclusive that the twin gene was from my nephew or from his wife.
But later in time, another person in my family tree, a daughter to my oldest sister, grew up and married and just had twin girls.
Now I think it is obvious that from the fact that twins were born to two parents, one male, the other female, who are both children of one or the other of my two older sisters, it seems highly probable that the twin gene comes either from my dad or my mom. It is difficult to know if either my dad or my mom had twin sisters or relatives who had twins but it seems to point to the possibility that either my mom or my dad had a predisposition in their DNA to have children who might one day bear twins.
So far both sets of twins from my two sister's children were both of the same sex. Both my nephew and my niece had twins but I am not sure if my nephew's twins were girls or boys. I do know however that my niece's twins were girls.
The possibility of there being both twin DNA in my brother-in-law's genes from both sets of parents, one being from family F and the other from family E is difficult to pin point.
The D gene and the P gene would be the best place to start to try to trace my ancestry as to the possibility of either of my parents having a special DNA type that produces twins.