Family tradition aside, it's always tricky when a friend or relative uses a name that you wanted to use, isn't it?
I wouldn't mind in the slightest if a friend or relative wanted to use the same name for their DC as we had. (Though it probably wouldn't occur to me that they wanted to!) But I suspect some friends and rellies would be very put out if we 'stole' their DC's name.
Having said that, there was a DS name I was quite keen on, but it was the name that a friend had given to her stillborn preemie son :( so I asked her how she would feel if I used it. She said, 'Goodness, llandb, I don't have a patent on the name! Obviously I think it's a great name and you should use it if you agree!'
And another friend caught wind of the fact that we'd been rather keen on the name she gave her DS - so she said rather frankly that she had twice been under family pressure not to use the name she wanted, knew how that felt, and that if we wanted to use the name, that was fine by her.
Is there any chance you and your brother could agree that both of you can use the name?
Otherwise, well, he hasn't exactly called dibs in any way that gives him priority over you (as you both 'staked your claim', as it were, simultaneously). And your present bump will arrive before any that they may or may not have on the way. So if your bump turns out to be blue, then I think you would be perfectly within reason to name him Sam :)
Just in case this helps you experience any potential guilt in a different light :) - would you feel the same way if your newly-married sibling was a sister rather than a brother? Or, if the tables were turned, do you think he would give up the name he wanted for your benefit?
Good luck!