Think about it like this:
One day, your mom and yourself walks along the road; holding hands, singing and everything is perfectly fine. You've been living together for 17 years now, never been away from each other for more than 2 days at most.Then all of a sudden some dude you've known since birth runs up to you with evidence and says "that person there really isn't your mom, she killed your mom and took her place when you were still very young". Even if you don't believe him at the time you might do so when you look up (down) at your mom's face and see her have the same reaction Yuno just had.
How would you react? I would probably react the same as Yuki.