I'm sorry you lost his brother.
What about getting him neutered when he's old enough, getting the females fixed, and reintroducing him to his mother and sister? I don't know if you know this already, but the high cancer rates in unfixed females makes it pretty imported to get the girls fixed too, even if you don't end up ever having them around another male.
Even when they are fixed, it's usually easier to create male/female bonds than it is to make same sex pairs. I wouldn't introduce an intact male to another male. If you want another male, I'd wait until both could be fixed to start introductions.
He should be ok until he's old enough to be fixed. Give him plenty of attention. Maybe allow him to still see and smell his mother and sister without giving them direct access to each other.
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