What are the chances of a brother and sister having twins but they not their parents twins.?

It is impossible for a brother and sister to have twins unless their parents are twins. Identical twins occur when a single fertilized egg splits into two separate embryos, resulting in two genetically identical individuals. Fraternal twins occur when two separate eggs are fertilized by two different sperm, resulting in two genetically distinct individuals. In either case, both twins share the same biological parents. Therefore, it is not possible for a brother and sister to have twins unless their parents are twins themselves.