what to do when kids get moody?

nothing. do nothing is the best thing to do.
today the sister-brother couple fight again because sister don't wanna share her book with him. at first the sister pushing all stuff away and started to cry. Followed by the brother, get moody and cried too. I tried to comfort them and explained to them. nothing helpful, she even cried louder; he was pushing me away.
Never mind, i said to myself; my voice can't 'fight' with their cries. Just let them be. Definitely my class was having disturbance.
Miracle happened a minute later... both of them stopped crying and played together again!!