I think he was just a companion to the grandfather, just a friend so he wasn't alone. (Though I haven't reread the beginning.)
With the uncle I'm pretty sure the uncle just seen mitsuki in a dream a few times but he couldn't forgot the message of being with the one you love.
I think Mitsuki is more like a Guardian Spirit/companion to most people and only was in a real relationship with the main guy.
"Scientists have discovered a jellyfish which can live forever. Turritopsis dohrnii is now officially known as the only immortal creature. ... It turns out that once the adult form of the 4.5 mm-wide species Turritopsis dohrnii have reproduced, they don't die but transform themselves back into their juvenile polyp state."
That's litterally what a certain jellyfish does. Mitsuki is just magical and can also turn into a person.
I am confused.Can anyone explain?Did Mitsuki low key date the whole family ooor?