I really don't want to step on any toes, but I have to echo GoodWill's research: it's not an accepted fact at all that identical twins are genetic.
"The frequency of identical twins across the world is approximately one set per 250 births and is generally not dependent on race, heredity, age or parity (number of pregnancies). "
The above quote reflects the most commonly accepted theory -- basically that having identical twins is completely random. I understand that in your family you were blessed to have two sets of identical twins within a few generations of each other, but that's not typical.
You are correct that fraternal twinning can be genetic because the tendency to hyperovulate can be passed from mother to daughter.
I guess I want to say that it doesn't reduce the specialness of having your girls born on their relatives' birthday! Just because identical twins aren't considered genetically dependent doesn't mean that a Higher Power didn't decide to connect the two
Alaska