In Unbelievable Episode 7, with two related suspects, Grace and Karen explored the possibility that they could be working together.
In the previous episode Netflix’s Unbelievable, Karen gathered DNA evidence from someone they thought was Christopher McCarthy, but Grace met him at his door.
Karen and Grace now had two suspects, Christopher and Curtis McCarthy.
With the Y-STR DNA tests, the sample Karen retrieved from Curtis would work to identify either of them.