Pool status matters, but there is still luck of the draw. E.g. if there are more entrants with SuperA status for a zone than their animals in the quota, i.e. a zone with a quota of 37 animals gets 38 or more applicants with SuperA status, somebody with SuperA still isn't getting a tag.
And the odds don't know that your brother-in-law didn't get drawn six times before while SuperA. If he applied for the same zone as your nephew this year he had no better chance than your nephew.
But to answer your specific question "Does anyone know exactly how the draw works?" - I don't, and I'm not sure even the people who operate the draw know exactly how it works.