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.