You can log into your iCloud account without logging out of your wife's account: but your Mac will only sync with the iCloud account which was entered there first.
'About Secondary iCloud Accounts
Secondary accounts should only be set up if you already have Mail, Note, Contact, Calendar, or Reminder data in another account that is separeate from your primary account. The steps in this article should not be used to move data from one iCloud account to another. For more information on how to move data from one iCloud account to another, see this article.
- Your primary iCloud account is the first iCloud account you sign in to on your computer or device. This account that will appear in Settings > iCloud on your iOS device, or Apple () menu > System Preferences > iCloud on your Mac.
- Your secondary iCloud accounts can be set up and viewed in Settings > Mail, Contacts, Calendars on your iOS device, or Apple () menu > System Preferences > Mail, Contacts, Calendars on OS X Lion. You may choose to do this if you have a second iCloud account and need to access its Mail, Contacts, or Calendar data.
Note: While you can configure multiple iCloud Mail accounts in iOS, Push mail will only work for the primary iCloud account in iOS. Secondary account(s) will check for mail based on the settings in Settings > Mail, Contacts, Calendars > Fetch New Data.'
If you want to make your account the primary account you will have to remove your wife's login from your Mac, which will delete any iCloud data from your <ac (but not from the server) befoe adding yours.