How would you compare and contrast the relationship of Oliver and Orlando with that of Rosalind and Celia?

The most obvious contrast is Oliver's hatred of Orlando compared to Celia's loyalty to Rosalind at the beginning of the play.  Despite being brothers, Oliver hates Orlando, in part because Orlando is better loved than he is.  In explaining his hatred in 1.1, Oliver says Orlando is "so much in the heart of the world and especially of my own people, who best know him, that I am altogether misprized."  Celia and Rosalind, on the other hand, are as close as sisters, even though they are only cousins.  Duke Frederick even says that Celia is made to seem less important as long as Rosalind is in the court (1.3 "She robs thee of thy name, / And thou wilt show more bright and seem more virtuous / When she is gone."), but that does not move Celia, who is so devoted to Rosalind that when Rosalind is banished, Celia considers herself banished as well.  While Rosalind, Celia, and Orlando take refuge in the forest of Arden, Oliver remains at the court and is coerced by Duke Frederick to pursue Orlando.  When Orlando saves his life, Oliver has a change of heart.  While he is no longer actively working against Orlando, Oliver never expresses the sort of affection for his brother that Celia holds for Rosalind.  In the only onstage moment where the brothers talk together after their reconciliation (5.2), the topic of conversation is Oliver's relationship with "Aliena," not their feelings for each other.  Much of Celia's character, on the other hand, revolves around her love for and loyalty to Rosalind.  While Oliver has other priorities and concerns--his estate, his status, even his love for Aliena--Celia is always centered on Rosalind.