The first panels have been uploaded here previously. New part starting from panel 18.
18) Yu'? R to L: munya...[mumble] mommie*... I don't want you to die... Reimyu* will* work hard to keep from poo pooing for mommie*... So take it easy*... munya...[mumble]
19) R to L top: Little ones...! Reimu's children are kind children who are really yukkuri...!! They're Reimu's treasure! No matter what happens from here on, I'll protect them...!! R to L bottom: yu' ru-b ru-b <3 yuu...
20)R to L mooornin' manjuus / it's time for breakfast / wake up- oh, from this early on there's poo poo on the floor already and one of them is dead
21) So, I'll just crush one right now so you choose which one
For example, in the removal_of_marisa's_accessory comics, the one marisa koyukkuri was able to take the place of the other without the mother or the reimu koyukkuris noticing.
^ Also, that's the basis of jokes involving mothers forgetting who their children are and adopting new children just by calling. Yukkuri aren't known for their exceptional counting ability.
In the particular case you mention, it might have been more "this is my daughter" and "this is my other daughter" to the mother, since that is what the first yukkuri originally was. The mother, rather, never realized that one was missing.
Being able to count the number of children is different from being able to distinguish them.
They might be able to tell apart different individuals based on something we can't sense. I don't think we've ever seen a comic where two individuals are confused as being the same by another yukkuri.
The cases mentioned before, it could simply be very bad memory and counting skills, not inability to differentiate.
^Still, as far as the mother was concerned, one marisa koyukkuri was as good as another. As far as yukkuris go, they can differentiate between types, but that's as far as it goes.
So, as far as this one goes, it has two choose between two koyukkuri which are just as good as the other.
many year ago in my country , a criminal took a children and cut their leg or arm and "DO" somthing that make their can not remember by their parents , then send to do a begger work. animal can forget their children because of the smell are changed. human can forget their children because of their children has been get tranformed to a strange shape.