Anyhoo: the given situation is that the more Ayase makes a mess of her life, the more Koharu will feel attracted to her. That's why letting Koharu into her pigpen of a room is actually a good move romance-wise. Same with her binge drinking and her inability to provide herself with proper meals. The more Koharu feels Ayase can't survive without her, the more Ayase will worm her way into Koharu's heart and the closer we'll get to a romantic happy end.
There's a manga I know with a similar plot, I don't think it's found in Dynasty. It's called Mai-chan no Onee-san Shiiku Gohan.
The loli Mai always has to take care of the beautiful and sexy blonde oneesan next door because this woman is unspeakably useless beyond belief and it leads to a giant crush: before Mai knows it she's so crazy about the oneesan she can't be happy without her anymore.