It kinda surprises me that they'd 'take care" of Reimu while still in the cave with Yukari. No more surprising than the fact that these parents are highly intolerant of shithead behavior I guess- most of the time it seems as though Yukkuri are blind to their childrens' own flaws.
Usually yukkuris tends to be shallow, combine that with stupidity and you get a creature that may forgive and not punish a little one just because "it's so cute and easy", of course there is a limit to that, and it's not necessarily a general case, since many yukkuri parents do react like the ones of this story.
Maybe their first batch of kids were also a Reimu and Yukari, and there was a similar situation to this, except that the Reimu killed the Yukari while they were away. This time the parents caught the Reimu bullying the Yukari, and decided not to take any chances?