The original question is whether the petition becomes void when her brother gets married. As I said, the law is silent on this issue, which means it's a grey area. If someone claims the petition is void, they should provide a reference to the relevant legal rule.
Regarding the brother's visa situation, if he was still married during the interview, he would not be eligible for a visa. However, since he is already divorced, his marital status at the interview will be unmarried.
The law could certainly benefit from an update to address situations like this. However, as we know, US immigration law dates back to the Immigration and Nationality Act of 1965 (INA). Back then, waiting lines for visas likely weren't a major concern!