If two siblings marry two first cousins, their children would be both first cousins and double first cousins to each other. Let's break down the relationship:
- Siblings A and B marry Cousins X and Y, respectively.
- The children of A and X would be first cousins because they share a common parent (A) and a common grandparent (X's parent) with X's children.
- The children of A and Y would also be first cousins because they share a common parent (A) and a common grandparent (Y's parent) with Y's children.
- The children of B and X would be first cousins for the same reasons as in point 2.
- The children of B and Y would also be first cousins for the same reasons as in point 3.
Now, since A and B are siblings, their children are cousins as well. Additionally, since X and Y are first cousins, their children are also first cousins to each other. However, due to the unique circumstance of the parents being siblings, the children share both sets of grandparents on each side. This makes them double first cousins.
In conclusion, the children of two siblings who married two first cousins would be both first cousins and double first cousins to each other.