Fuck to those two fucker

It was not those two that killed the sister. In the last panel you see a corpse on the floor witht a gun. It's the one those two soldiers killed, an enemy I guess , and that's why those two guys don't kill the brother in the wheelchair. They save him in killing the one on the floor, but they weren't in time for the sister sadly

https://www.mangago.me/recommend-manga/sea_foam/
(Ilas I see you have put that comment a while ago, I'll put the link again there)