Your text isn't clear on whether or not you are about to invite your wife to your iCloud+ plan or have already done so.
However, this YouTube video is a very clear demonstration of how to invite someone to a Family Sharing group, and subsequently how the new member can adopt the group's iCloud+ storage space. If the new member has an existing iCloud storage plan (including the free one) then they have to confirm that they wish to leave the existing plan and join the family plan.
At that point the billing for their old plan ceases and the new plan is adopted.
Just check that in adding new members to the shared iCloud+ space that you subscribe to a plan large enough to accommodate all members' existing storage needs - and have more for future growth.