Originally Posted by Major Kong
I voted no, but I must add since they are not blood related I do not see a problem. Before anyone accuses me of having no limbs in my family tree let me explain. What your describing is fairly common place in rural/lightly populated areas of the country. As I've mentioned before both my Dad and Mom's family are from extremely rural areas of Appalachia. If anyone is familiar with the term "double first cousins" my family has several.
My main concern would be this "Also, she is 19 and he is 27 with a kid and engaged." That would be what I would worried about.