Can my half-brother obtain Thai citizenship after our father’s passing?

The way it works is; if one parent is thai no matter where a child of that person is born they're thai too.

You'd need to start with your dad's family, getting his passport, his thai i/d, his house book listing, the marriage certificate to the half brother's mother, the birth certificate showing him as the father. Your brother is definitely thai by birth because his father was thai. Its just getting the documentation to prove it.

I certainly sympathize with your plight BUT

Unfortunately it's not a thai visa related question so I have to close the topic, :/

Good Luck,

You’ll need to start the process in Thailand by organising a DNA test with you and your remaining Thai relatives to prove the link.