@ciera : Younger one is the one that's sick right now.
Older one...well, if she's lived a life of being ostracized for being 'impure', then she's probably the prime example of 'beware the smiling ones'.
So if I understand this chapter right.
The little sis got shot by a gun. Since she doesn't seem to have any signs of actual injuries, probably a past event and not from guns stolen from Souya.
And her sickness is probably lead poisoning from the bullet still lodged in her.
Personally, I'm hoping he save her. Not because she's elf waifu to be saved but for his own heart.
Because he too have a younger sister who's still sick only because he didn't have the money, because society was a cold, heartless place.
So if he abandon her here, he will have chosen to become something closer to the society that abandoned his sister.