According to her other post, both siblings were born HK, so they were HK Chinese citizens as their parents were also Chinese citizens.
I think your brother needs to get some serious advice, he doesn't sound like he knows how to handle his matters in HK and China.
Based on what you said in the other post, he probably needs to do a VEPIC, and that requires the person to be physically in HK on the application, so he should go to HK again to apply it, it should be trivial if he was a Chinese citizen born in HK.
His daughter is essentially a mainlander, and will need a one way permit to settle in HK (based on what is described should be a HK permanent resident with ROA). Or a two way permit to visit. I am not sure how the US passport will come to play as Chinese citizens are not normally allowed dual citizenship (with exceptions for HK/Macau, though I guess with a HK parent she can considered a HK resident)
Even though your brother is also American, he is exactly the same as all the other HK men who have a mainland spouse, expect lots of waiting.