Prior carrier probability = 2/3 (for a lethal mutation if male and female mutation rates are equal).

In this example, only few non-affected boys reduce significatively the risk.

The pre-test risk for the first cousin is low (1/77 or 1.3 %) and less than 1/1000 for the first cousin once removed.

These low risk are to be compared with the general population risk of being carrier of a DMD mutation, that is about 1/2500.

Risk can be further lowered with a simple (non-DNA) test.