Chen's wife Reimu gave birth to koMarisas. Dumb whore Reimu doesn't notice the problem, but sharp-minded Chen immediately realized that Reimu is cheating on her with the neighboring Marisa. To add to this Chen found out that Marisa is actually an arrogant homewrecker who brags about refreshing with Reimu behind her back.
Quite bold of Marisa and Reimu to betray Chen's trust like this condisering she's the strongest yukkuri in the neighborhood. Well they're promptly slaughtered.
Fortunately for the kids, Chen is a nicehead with a heart of gold so she decided to raise the koMarisas as her children anyways.
Part of takumi's Abyuse Oneshot series (pool #1266), a collection of several short abyuse comics. I'm giving some of the shorts from there their own pool to make it easier to navigate to them for rereading purposes.