Like many families of royal and noble status, intermarrying was apparently a thing amongst the Rothschilds. Cousin marriages were not uncommon for centuries, it was a way of keeping royal / noble / financial statuses stratified and contained. Mayer Amschel Rothschild, the founder of the family, clearly fell in line with this way of thinking. He arranged things so that the female members were limited in whom they could marry… if they wanted their inheritance. If they wanted their vast inheritance, they had to marry their cousins. From Rothschild’s perspective, it was the best way of keeping the money in the family and out of the hands of outsiders.