Not necessarily. She could have artificially inseminated herself.
But again, the FLDS probably wouldn’t want to use that defense.
Then the second step is proving the “marriage” between Mahlon and Sarah. One would hope prosecutors can find some sort of religious or civil documentation of the marriage. So even if Mahlon says it was his brother Levi that impregnated Sarah, I believe that children born during a marriage are presumed to be fathered by the husband.
Plus the question was, “Why do DNA testing?” You gotta start somewhere.
Another plus, aren’t the polygamists the Alpha Males in the group- and then the others get banished? I am not sure that Levi, Zebulon and Eli are still around to get blamed.