Question On Fallout 3. Possible Spoilers.

[Oniichan]

Dont kill him as it will affect a sidequest that i believe gives achivements/trophies.

If you dont want to kill him then when Amata offers you a pistol just tell her to keep it (she gives it to you later). Then when you see her in the room with the guard and oversear just watch. Dont interupt it.