I've loved this story soooo much. I just kinda wish things had resolved better with the sister, there's nothing wrong with just texting, but I guess the figurative bits of that ending make sense. So cute, so up and down, such a ride and so worth reading.
I have family members like the mother and sister. I, too, tried to be good to them and give them my love and support, just like Ewon. They sucked me dry of my life, my will and all my energy. I was so busy caring for them, there was nothing left even for me to care for myself. Some parasites drain the host until death and then move to the next. The mother definitely is this kind of emotional parasite and the sister will become this way if she does not learn to be strong and independent soon. If the sister grows up the way Ewon is giving her a chance to, she will grow strong and independent. Always relying on others makes us lazy and unable to live with strength on our own. I think, after a few years of independence, Ewon will be able to visit without fear of being manipulated and drained by their neediness. He can't text because text leads to a call and a call leads to a visit and then they are right back where they started. He did not abandon them, he provided for their care. He is blameless. This is how things went for my family members. After three years of no contact, I was able to visit without fear. We all had grown stronger. It's "tough love".
In many ways that is true, toxic family, despite being family, cen still be toxic people that should be kept out of your life if they never change and only end up draining you dry. I guess things will be different in a few years for the characters, and even more so when the sister is eventually on her own. When (and if) that ever occurs, he could evaluate if she is a fair person to have in his life.
There is just such a finality to their last encounter, as if he will never see her again. If it happens as you describe it, where once his family gets stronger, especially the sister, he can see them again, that would be fantastic. Just the way it feels in the manga is that he is cutting out his twisted past, including his family, almost completely. Not abandoning them, as you said, but not involving himself with them at all. Still, all in all, gives a decent ending to it all, with the loose ends involving the family tied up and us knowing the family isn't doomed because he made arrangements for them.
And I should also say I am very glad you pointed this all out. Although I have mostly avoided family and people that are parasitic, I have seen the effect they have on others. My father is one of those who tries to take care of all the "lost sheep" of the family and it ended up tearing him apart, almost literally. Ewon would probably be far less of a person due to their reliance on him. Even though him and his lover have a very obsessive and possessive love, it in the end has proven healthier for the both of them.
Kinda like a silly play where one of the main characters didn't get the full script change and is still trying to catch up. Hilarious, funny, kinda cute and absolutely flamboyant.