I'm going to do what I can to leave all of them alive. I just can't bear such an idea. If I had to, though, then I would have each sibling defeated by the little sister of the other country. (In other words: Marx, Camilla, Leon, and Elise would be executed by Sakura while Ryouma, Hinoka, Takumi, and Sakura would be executed by Elise.) It kinda sounds sick, but I'd imagine that each youngest sister would have genuine sympathy for their "victims", knowing that they also loved Kamui (save for Takumi and perhaps Leon, but Elise/Sakura would still act without malice regardless).
I mean, think about it. Let's say that Kamui sides with Hoshido and Hinoka kills Camilla. Wouldn't it be absolutely ghastly for Camilla's dying thoughts to be "I lost... to Kamui's real older sister"? It's been made clear that she loves Kamui a lot, and I don't see Hinoka - who would view her as a thief - to show much sympathy for her. Or put it in reverse and have Hinoka get killed by Camilla. "I lost to that girl who stole my baby sibling and made herself his/her big sister." I can't see Camilla being sympathetic to her in any way, either. It'd be the same with just about everybody (save Takumi).
If I took the Hoshido Path, then I would choose to be executed by Marx.
If I took the Nohr Path, then I'd choose to be executed by Takumi. He seems the least likely to lose sleep over it.
Bad choice on the last one. Camilla's got Axes while Oboro has Lances.