Rivalry

Sibling relationships are naturally ambivalent, because they're a mixture of love and hate. This is because of sibling rivalry. Brothers and sisters are very close, belonging to the same generation, and form an alliance against adults in general and especially against their parents. However, they're also in competition with each other because they fight for supremacy in the family group (even if the group only consists of two) and their parents' affections.  

Thankfully, the fight for supremacy doesn't last forever and it isn't linear: each child has his turn at being the one in favour for different things, or they can both be in favour at the same time. For example, one might get the new toy they wanted, but the other might get served first at dinner, or one might be better at sport and the other better at spelling, etc. 

What seems like an intense hatred that emerges for no real reason disappears as quickly as it appears because of fraternal love. One minute they're tearing each other's hair out, but the next minute they're laughing and playing as if nothing had happened.