My oldest son had to have a bone marrow transplant and my youngest daughter was an identical match per the doctor how could that be?

Identical twins share 100% of their genes, including the genes that determine their HLA (human leukocyte antigen) type. HLA type is essential for bone marrow transplantation, as it determines whether or not the recipient's body will accept the donor's bone marrow. In order for a bone marrow transplant to be successful, the donor and recipient must have the same HLA type. Since your youngest daughter is an identical match for your oldest son, they must be identical twins.