Didn't know whether it ate Reimyu's corpse or not, but based on the remaining accessory/braids of Reimyu, it almost guarantees that she scavenged the corpse of her sister.
Look at the most-right part of the second post (post #80350), Reimu is crushed by a car/bike/vehicle, and died. So now they didn't have anything to eat, just kept waiting and waiting for their mother to return. And finally, died from starvation.