Add wife's iCloud account with mine on iPhone 6+

You can add her iCloud account here: Settings > Passwords & Accounts > Add Account

Note that you will then potentially have access to all of her iCloud services, not just her Calendar.

You may want to consider the following: Share iCloud calendars on iPhone - Apple Support

another option -- also see this from that link:

If you set up Family Sharing, a calendar shared with all the members of your family is created automatically. See Share photos, a calendar, and more with family members on iPhone.