I translated this... and I'm still not sure I even get it.
Is this sibling jealousy? The fem-anon resented her younger sister, then the younger sister died (killed by fem-anon? disappeared?), then fem-anon turned to abyuse 'cos the wasa reminded her of the younger sister so much?
After closer inspection (thank you for translation, BTW!):
Something happened to her little sister (death, possibly? Divorce?).
If you notice, she's wearing a Reimu ribbon. It's probably this wasa's big sister's ribbon.
As you know, in some lore, yukkuri -completely- identify by what accessory is worn. If you take a Marisa's hat and wear it, the Marisa's wife and kids might wind up looking at you as if you're the Marisa husband/father.
The small Reimu completely believes the girl is the big sister (thus why the constant references to her younger sister).
I'm not sure if this was going somewhere, as usually these stories tend to be only a fraction of a story, but yeah. That's what I gathered.
Edit: That's why sometimes, when the view flips over, you see a larger Reimu. That's how the wasa views the girl, since she's wearing the ribbon.
Edit 2: "Wrrryy is shitty human here?!" She's holding the ribbon in her hands.
No wonder it felt so weird. This pool is a sequel :v
In the prequel, fem-anon's little sister had died and fem-anon had already resented her. One day in a yukkuri shop the fem-anon saw wasa Reimu, reminded of the little sister, and bought her. The accessory fem-anon had was actually given by the shopkeeper.
So yeah, I guess fem-anon was jealous of the little sister, but the sister died before fem-anon could torture her? (or maybe it wasn't enough) So wasa was bought as the replacement. Or something. Rainbows and sunshine this pool sure isn't.