As someone with a twin (a fraternal twin in my case, and we barely look like brothers, much less twins), I have always been a bit squicked out by twin/incest fetishes. In this case, Jack and Finn might be the exception.
Does anyone know if they are identical or fraternal twins? They look similar enough that they could be identical, with the differences in appearance more related to grooming styles than genetics, but different enough that they could be fraternal. (They could be "polar body" or "half-identical" twins: one egg that split and was then each fertilized by separate sperm, so their genes from their mother could be identical, but the genes from their father could be as different as any two siblings.)
In any event, my "no brothers together" rule would be something I would make an exception for here. I would be thrilled to have any sort of erotic encounter they agreed to, whether it was simply seeing them naked or full on sex -- and I'd be happy with either or both, separate or together, in whatever manner they wanted.