Think about this, even if it's possible would this author really do something like this, just "kill" her? Not only doesn't makes his style, also, just because she is "ill" doesn't mean that she'll drop dead suddenly. A chronicle disease isn't like that, no matter how many anime and manga clichés may make you think otherwise.
I believe that Magari also is not expecting to die suddenly, what she must expect is a shorter life expectancy, and that's that. Before dying she will first feel unwell and be admitted on a hospital, from which she may recover again. Instead of Nakami suddenly losing her what will probably happen is that he'll have to live with this reality that he'll probably have to see her dying slowly.
Looks like he already takes care of himself and even his father a bit.