An ad hoc version contributed by Jasper Griffin via Julia Griffin and Jürgen Gutsch:

Si virtus tenera floret pulcherrima forma

iste niger non est: candidus, ecce, gener.

If the power of the most beautiful blooms in a tender form

This son-in-law is not black: [he’s] white, look.

An ad hoc version by Prof. Dr. Wilfried Stroh (Munich University) via Jürgen Gutsch:

Si pulchra una bonis virtus probitasque videtur,

pulchrior hercle magis quam niger iste gener.

Back-translation by Jürgen Gutsch:

If real virtue and honesty count as beautiful for good people,

Then, indeed, your son-in-law is far more beautiful (= white/fair) than black (=ugly).