We are not in the business of doing your homework for you here, so you'll need to write your essay yourself.
However here are some points to consider about Viola and Olivia:
Olivia is similar to Orsino in that she can be viewed as very self indulgent. She locks herself away after her brother died for a long period of time. Once Viola/Cesario starts visiting her however, she decides to put aside extended mourning and instead get wrapped up in a melodramatic pining love sickness for Viola/Cesario. Olivia (like Orsino) is unbothered by the fact that Viola/Cesario does not return her affections and is content to attempt to wear her down with enough time.
Viola is an interesting study in gender roles. She starts out in the play as female gendered but then is forced into a male gender role in order to take care of herself after the shipwreck when she joins Orsino's household dressed as a boy. By the end of the play, it can be argued that she has transcended gender and is neither male nor female per se but instead can take on parts of both gender roles.