There are any number of opinions you can form that you would like to argue (prove) about the relationship comparisons and contrasts regarding Oliver and Orlando as compared to Duke Senior and Duke Frederick (such an opinion and argument/proof is called a thesis statement). One possible one is that Shakespeare is exposing the truth that covetous hatred can come from younger generations as well as older generations.
The major points of comparison/contrast between Oliver and Orlando to Duke Senior and Duke Frederick are that the former (first listed) are an older brother mistreating a younger while the latter (second mentioned) is a younger brother usurping the power of an older brother. Another is that Oliver tries to kill Orlando while Frederick exiles Senior. Similarities are that Frederick and Oliver are both wicked while Senior and Orlando are both noble.
Your conclusion will simply give a brief statement of what you proved in your essay and suggest what other idea could still be explored related to their relationships.
Oliver is Orlando's elder brother. Both are the sons of late Sir Rowland de Bois. In the very beginning, we find out, in accordance with the rule of primogeniture in England, which instructs that all the property would be inherited by the eldest of the family, Orlando gets the possession of the belongings that his father leaves. Though, he was instructed to take proper care of Orlando, he, out of jealousy and loath, makes Orlando totally deprived. In fact, when the royal wrestler Charles is supposed to fight Orlando, Oliver leaps with joy thinking of the probable death of his brother. But, in Orlando, we find no such cruel feeling or motive. He is not even confident enough to stand on his own feet. He just flees away in fear of his brother.
Later in the play, when Oliver is attacked by a lioness in the forest of Arden, it is Orlando who rescues him. Orlando could have shown abhorrence or reacted vindictively by not saving Oliver, but he does not do so. Unlike Oliver, he proves to be a sensible brother and a good-natured human. And, at this point, his valour is also expressed.
Another notable point is that, Orlando's character develops gradually with the help of Ganymede, whereas Oliver's character faces a sudden change in the forest of Arden which is very miraculous.
The difference between the two brothers has clearly been depicted in the play, where Shakespeare's portrayal of Orlando is almost a mirror image of Oliver.
Oliver and Orlando are the sons of Sir Rowland De Boys. Both have a blood relationship but desire nothing but the blood of each other. The are bitter enemies, be it in monetary or social terms. Similarly, the Dukes also have a strong hatred towards each other. Even here, monetary terms(estate and kingdom) comes into play.
A very interesting thing is felt at the end. Both the villains - Oliver and Duke Fredrick, renunciate public and courtly life and prefer to live in the forest, ceding all their property to their brothers.
The nobility between Orlando and Duke Senior is also similar although the Duke has a feeling of pride in him.
These are the basic points ..... Hope it helps !!!