When does Brotherhood and the 2003 series split?
Episode 12 in brotherhood is where the story changes from the original series.

it a bit later in the original series, episode 17 or 18.

The key point is the death of hughes, that's where the story's split.

the original series stretches the backstory a bit compared to brotherhood so its worth watching from the beginning but you wont miss anything important if you start at hughes death.