@ThrystaVindr I still don't think her being fresh out of middle school makes it all that different. I'd obviously be against them marrying while she's still in high school or so, but I say dating is fine and they can marry once she graduates. And life experience really isn't the factor for whether or not a relationship will work or not. You can have all the life experience in the world and be a shitty partner or still immature, experience doesn't mean shit if you don't learn from it. I think she has plenty of life experience regardless with her mother dying, the situation with her father, helping out around the house with her sister, etc. Just because Kiriyama has a lot of life experience doesn't mean that she doesn't either, and if anything both of them have a fairly immature side to them so you can't really make the argument that Kiriyama is just way too mature for her or something like that either.
So in the end, the only real main issue for you is that you can't see them as romantic partners, which is fine. That's your opinion and neither side is wrong it's just an opinion on whether or not you think they do work out as romantic partners or not. I personally think they can definitely make romantic partners. Someone you spend all that time with, watching them from the side, living life together, seeing their vulnerable moments and their best moments, I think all of these things can build a level of intimacy that can go beyond just a familial sense. If you're stating that they're too much like family is the biggest reason, then all childhood friend tropes should never work out. Being like family isn't necessarily something that can't be progressed beyond, I don't necessarily think that it's impossible to make the relationship romantic because they're as close as family.