G: Yes, generally.

If both iPhones are signed into the same Apple Account and both have Find My enabled, you can use one phone to locate the other.

A few distinctions matter:

• Find My iPhone (the tracking feature) is normally turned on during setup when you sign into your Apple Account and enable Find My.
• From Phone A, you can open the Find My app and see Phone B if Phone B is associated with the same Apple Account.
• You can:
  • View its location.
  • Play a sound.
  • Mark it as lost.
  • Get directions to it.
  • Erase it remotely.

If by "activate Find My iPhone" you mean turning the feature on remotely after it was previously off, that's different. Apple generally requires access to the device itself to enable Find My. You can't usually flip it on remotely from another iPhone if it was disabled.

So the answer depends on what you're trying to do:

1. Locate a second iPhone that already has Find My enabled? Yes.
2. Turn on Find My remotely on a second iPhone where it's currently disabled? Usually no.
3. Add a newly set-up iPhone to the same Apple Account and have it appear in Find My? Yes, once it's signed in and Find My is enabled on that device.

If you're trying to recover a lost phone or set up a replacement phone, tell me which situation you're in and I can walk through the exact steps.

G: Short answer: maybe, but only if the account setup is even more intertwined than you think.

The first thing I'd check is this:

• On your iPhone, open Find My.
• Tap Devices.
• See whether her iPhone appears in the list.

If both phones are effectively signed into the same Apple Account for Find My purposes, you may see both devices there. If so, you can tap her phone and see its last known or current location. citeturn0search17turn0news25

The second thing to check:

• In Settings, tap the name at the top (Apple Account).
• See whose name/email appears.
• Then go to Find My and see whether Share My Location is enabled and which device is listed as Sharing From. citeturn0search1

A weird but common situation is that:

• The App Store is using one Apple Account.
• iCloud/Find My is using another.
• Family Sharing is involved.
• People aren't entirely sure which account is signed in where.

If that's the case, the answer changes completely.

Given the circumstances, I'd also check whether you can log into:

urliCloud Find Deviceshttps://www.icloud.com/find

using the Apple credentials that the phone thinks belong to your mother. If you can successfully log in and multiple devices are attached to that account, you'll see them there as well. citeturn0search2turn0search17

One caution: if you and your mother have been sharing a single Apple Account for years, the device list can become a mess. The fact that the App Store "thinks you're her" does not automatically mean you have access to her phone's location. Find My, iCloud, Family Sharing, and App Store purchases can all be configured differently. citeturn0search1turn0search11turn0search6

Before we go any further, tell me:

1. Does Settings → [name at top] show your email or your mother's?
2. When you open Find My → Devices, do you see one iPhone or multiple?
3. Is she carrying an iPhone, and do you know whether she normally has location services turned on?

Those three answers will tell us whether this is a 30-second check or whether we need to untangle Apple's account spaghetti first.

G: If you're asking, "Can I use Lost Mode as a probe to see whether my mother's phone is online and where it is?", then the answer is possibly, but only if you actually have access to her device through Find My in the first place.

What Lost Mode does when you trigger it:

• It immediately places a request on Apple's servers to mark that specific device as lost.
• If the device is online, it locks, disables Apple Pay, and begins reporting location updates through Find My. You can then see its current location and movement. citeturn0search0turn0search3
• If the device is offline, the request sits pending and activates the next time the phone connects to the internet. citeturn0search0turn0search14
• Apple sends confirmation that Lost Mode was requested for that device. citeturn0search0

The important practical point:

You cannot accidentally put your mother's phone into Lost Mode unless Apple already considers you authorized to manage that phone. To do it, the phone has to appear in your Find My device list, or you have to be logged into the Apple Account that owns it. citeturn0search0turn0search15

So if you're currently not seeing her phone anywhere:

• Triggering Lost Mode on your phone won't tell you anything about hers.
• Triggering Lost Mode on a device you don't see in Find My generally isn't possible.

Given the concern level here, I'd actually check something else first:

On your phone: Settings → [name at top] → Devices

Apple usually shows all devices attached to that Apple Account. If her iPhone appears there, that's a strong clue you're sharing more of an Apple identity than you thought. If it doesn't, the App Store confusion may be completely separate from Find My.

One other question: when you say "we have concerns," do you mean:

• she's not answering calls/texts,
• you think she may be traveling somewhere unexpected,
• or you're worried about a medical/welfare situation?

Because if this is a genuine welfare concern and she's a state away, there may be faster options than trying to untangle Apple's account structure.