Instead of going out hunting, Parent Reimu secretly went around to the back of the house. There, she removed some bean paste from Parent Marisa's corpse and mixed bitter grass, sand, and other uneasy things into it. This was to prevent little Marisa's taste buds from being spoiled--Reimu intended to feed this to little Marisa.
Yukkuris have a fundamental taboo against cannibalism. However, this only applies to pets and wild yukkuris who have food in abundance--it's different for strays. They prolong their lives by eating their dead relatives and friends. Even this Reimu had survived this long by eating her mother, father, and sisters. That was not at all unusual. That's how harsh life as a stray was.