It's probably just implied that the overall amount of stalk which comes from the parent is only barely enough for the Stalk-Babies to eat.
It's probably also implied that a Stalk-Baby requires the stalk as its first meal in order to develop properly, so that if it were for some reason not able to have the stalk as its first meal, its development could be stunted, or it could even die - this goes back to the first point in that:
If the mother Reimu had decided to share part of the stalk with the other (Mammalian-Birth) Reimu, the Stalk-Reimu possibly could have died.
I suppose that if you disagree, then you might say that it's actually the mother that should be ashamed of their actions, but the mere desire to satiate one's cravings (i.e. the Mammalian-Birth-Reimu wanting to eat the stalk, for absolutely no reason other than that it was a “Schweet-Schweet”) should not be justification for raping, killing, and ultimately cannibalizing the body of her younger sister.
There might be another case where the mother would be at fault, say for example, if the mother had only a limited amount of food and decided to give it all to the younger Stalk-Reimu, and did it out of favoritism and thus starved her older daughter (the Mammalian-Birth-Reimu) for no reason other than that she just simply likes the younger daughter more, then you would have some basis of culpability. In this case, I don’t believe the mother Reimu did anything wrong - if a Mammalian-Birth baby does not require the First-Meal-Stalk in order to survive, then they do not have any basis for wanting to eat it – and if they do want to eat it, then it can only be for selfish reasons.
These are all just things which I'm completely making up out of thin air, but I try to read a lot of the Yukkuri stories from the context of the Japanese culture (since those are the actual people who are producing most of these stories which we even read about in the first place), where shame is a very imminent and predominant aspect of society.
It's probably also implied that a Stalk-Baby requires the stalk as its first meal in order to develop properly,
It'd been a minute since I last read any yukkuri stories involving stalk birth, but I'm pretty sure this is always the case. I remember reading an old anko that said something like: the stalk is extremely nutritious, and while it taste sweet, it doesn't too sweet to the point that it ruins the koyu taste bud, so effectively both good for their bodies and their minds.
Of course, it wouldn't be a yukkuri works if the rule lax as heck. Like post #83155, the koyu still survive and develop normally even though they didn't eat the stalk.