A Faithful Servant

The gemara says that Moshe couldn't see the tzaraas of Miriam and declare her impure because he was not a kohen and Aharon couldn't see the tzaraas because he was a relative and was therefore disqualified. Everyone asks - Moshe was also a relative so why didn't the gemara use that explanation as it did for Aharon?

Answers the Kotzker: Only in dinei nefashos is a relative is absolutely disqualified as a judge because Hashem is the baal din and nobody has the right to overrule Him. In monetary matters, however, a relative MAY judge if the two parties agree because they are the baalei din and have the right to accept anyone they choose as judge. A baal din can accept a normally disqualified judge.

The Torah says about Moshe "בכל ביתי נאמן הוא" - He was completely faithful to Hashem. This being the case he was not disqualified as a relative in dinei nefashos because a relative is a valid judge where the baal din trusts him. Hashem completely trusted Moshe making him a kosher judge even for dinei nefashos [and tzaraas].

That is why the gemara was compelled to say that Moshe was disqualified not as a relative but as a non-kohen....