Language / Etymology of the word "brat" - in Poland and other Slavic countries [21]

From Wiktionary.

The English brat:
en.wiktionary.org/wiki/brat

The Polish brat:
From Proto-Slavic *bratrъ, *bratъ, from Proto-Indo-European *bʰréh₂tēr.

Well, it doesn't explain much, but it won't be easy to find more accurate explanations.