This feels REALLY out-of-order (left-to-right? bottom-to-top?) but long story short (from what I pieced together): It's about a clan of yukkuri who live in the forest. Three children grew up together: A Marisa, Reimu, and Alice. Marisa and Reimu were childhood lovers. One day, Marisa's stick goes missing. Turns out the Alice killed Marisa's Reimu and replaced her bow. She knew once 'Reimu" gave birth to an Alice. Having discovered this, she killed Alice, then I guess since they thought Marisa killed both Reimu (since it was her stick) and Alice, they buried her (alive?) with her love's bow.
Reimus are legit victims all the time. It is just rare to find one actually deserving of sympathy. Most of the time, they do themselves in with their nature or stupidity. This one was flat out ganked.
I know this is late, but just to tie this up: if you look at the previous panel near the bottom right (next to marisa), you will see the covered corpse of alice. Reimu's death probably isn't even a factor here.