This is definitely one of the most disappointing works I've seen or read until now, though it's not like I absolutely hate the adolescent part, without it I might've given this a 6 or a 7.
Now, I can relate to the awkwardness of how to behave near a child in order not to set a bad example, in that regard it was done pretty well, the problem is that without it the writer doesn't have anything to show off, being (at least at the time) weak on most fronts.
I wasn't engaged, enjoyment was few and far between and I didn't learn much of anything at all. I consider most of my time wasted.
So here I'll give a long ramble of the top 5 worst parts of this (and possible solutions if I can think of one) from horrendous to least bad.

1. 10 year transition period
The idea itself was to begin with something that could only come from a brainfart, but even the execution is absolute garbage. Nearly nothing (aside from a few appearance changes) was different, no new friends for either the mc or his aunt, barely any changes in demeanor (aside from the overly obvious, like the brittle bones or that during puberty hormones become a thing) and on top of that the author throwing away the only thing (s)he (can't bother to find out) had going for it. A solution to this would either be a montage, or just a faster change in pace, like using 2-3 chapters to cover a year of growth.
It'd still be jarring but if you make good use of the panels you might be able to work out a natural transition.

2. Plot
So you have the big opener: Girl get's abandoned by everyone except for the savior mc, he has to learn how to become a good parent. Sadly the execution of everything after is boring as shit because a) very little happens, and b) the stakes aren't set up properly.
And you'd need at least one of them to keep a reader engaged. Now it's obvious that the author wants to write an SoL of the most average life possible, so A isn't going to happen. What about B then? Basically what could've been done is show how the little girl would be impacted by wrong behavior from the mc, to give him reason to stress over it. Also show (or at least give more than a vague indication of) the finances, have him try to pick out very cheap school supplies to cover for the month, show how much worse the situation became after changing departments because I sure as hell didn't notice any effect (aside from one joke). Basically set up hurdles for the duo to overcome, no matter how mundane or trivial.

3. Atmosphere
This is a pretty subjective topic, since it's literally about the feeling it gives off but what I noticed is that it was a greatly missed opportunity to make an average (at that moment) manga into something 8-9 worthy. What I anticipated what a sort of familial atmosphere which I guess the author tried to set up but came short of. I guess the major issue is that the main duo never try to rely on each other, or have a heart to heart conversation about how they feel about their situation. This makes them feel a bit too distant, not so much a father and daughter relation as him pretty much feeling like a babysitter. It felt pretty uncomfortable seeing as that was probably the entire point of the manga, seeing them grow together as people, before the writer wanted to go for a garbage romantic drama.

4. Romance
Spoilers lol
So basically the childhood friend guy really wants to date main girl but he's with another girl because of a plot contrivance which he can't get out of, so he says go away and the plot contrivances sister says no and thus something something wincest time but it actually isn't and the main character knew all along even though a few chapters before he calculated blood relation but no this is a morally upstanding manga so it's just going to be pretend-incest and they make a big deal out of it but probably noone gives a flying fuck.
This shit is just retarded, maybe a fix would be to just rewrite everything after chapter 26 (I think it was).

5. Characters
All I really knew about the young main girl was that she was a masegaki (kid with adult like behavior) and I knew from the mc that he barely had a will to live but now slightly more. I also knew that the young romance rival was reckless and dumb. After the 10 year interval I knew that mc needs some calcium, that main girl is just generic JK and that rival looked like he came straight out of a shoujo manga but was still a complete idiot. Just because they behave like people doesn't mean they have depth. Just give them some hobbies or a bit of personal philosophy, maybe a few quirks or fields of expertise. Not really that hard.

TL;DR
writer is very bad at writing.