Q: My sister recently wiped her iPad clean because she had forgotten her password. We now share an Apple ID account. She is now getting my text messages whenever she turns on iMessage from her iPad. How can I completely shut her down and be able to prevent her from getting my texts?
A: This is basically an inevitable result of sharing an Apple ID between more than one user.
Since the email addresses and phone numbers for sending and receiving iMessages are associated with your Apple ID, what you’re essentially doing when sharing an Apple ID is configuring both your own device and your sister’s to use the same addresses for iMessage.
You can configure multiple email addresses for a single Apple ID and then choose to receive messages addresses to only specific addresses on a given device. This is done by going into the Settings app on your device, and then choosing Messages and tapping on Send & Receive.
If your sister is already using a different email address that is associated with your Apple ID, she could simply de-select your address from this screen by tapping on it and she will no longer receive any iMessages sent to your address.
Note that this does not remove her ability to toggle this address back on, however, in which case she would once again begin receiving iMessages sent to you. While you should receive a notification of this on your device, in reality the simplest way to avoid this is to have her sign up for her own Apple ID—at least for iMessage and FaceTime use—since it’s completely free to do so.
Your sister can sign out of iMessage on her iPad simply by tapping the Apple ID shown at the top of the Send & Receive screen, after which she can proceed to create a new Apple ID of her own by following the instructions on the screen.
If your sister’s email address is already associated with your Apple ID, it will need to be removed from your account before she can associate it with her own.