HHow do I add my wife’s iPhones to the find my iPhone app?
How do I add my wife’s iPhone to find my phone?
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Your wife has to turn on sharing on her phone: Settings > her name (at the top) > Find My > Share My Location ON.
Then open Contacts app, tap your name in the list, scroll down and near the bottom tap Share My Location > Share Indefinitely.
On your phone, accept the request. You can tap Share to also share you location.
More details here.