I really think a romantic relationship between Shino and Nozomi would be a mistake by the author. The arc of the story is the formation of a family when the initial bond between them is traumatically torn away. At the start, the only goal either of them has was to not be an imposition on the other. Shino wanted to grow up and get a job quickly so as not to be a burden on Nozomi, who she suspected didn't want the responsibility. Nozomi, on the other hand, didn't know whether Shino wanted her in her life at all, much less be a caretaker.
Watching them grow closer to the point that they can demand of each other to not hide their problems has been very satisfying to watch. That there's still ambiguity on whether they are creating a older/younger sister bond or a mother-daughter bond is interesting (translator's comment notwithstanding). For this to switch to a romantic relationship would risk erasing the complex family dynamic for an easy way out of the story--especially if the story ends when the romantic relationship begins (like 99% of all romantic stories in all mediums).