5. Isabelle and Alec from the Mortal Instruments Series
4. Adam and James from the Shatter Me Trilogy
Adam Kent is forced to take care of his younger brother, James, because his father beat him senselessly and their mother is dead. They have more of a father-son-type relationship. I've only read the first book and the novella but I know that the relationship between Adam and his brother is a factor that landed me a spot on Team Adam (sorry Warner, I like you but...ADAM!). They're just so loving towards each other and it makes me happy because come on, it's adorable!
3. Katniss and Prim from the Hunger Games trilogy
2. Alec and Jane from the Twilight Saga
1. Fred and George Weasley from the Harry Potter series