How can I retrieve me and my sister’s messages?

Delete a message or conversation

When you delete a message or conversation, you can't get it back. So make sure that you save important information from your messages.

Though restore from iTunes backup is an option but you need to restore the entire iPhone (only messages restoration option does not exists) including all apps, Photos, Videos, Voice Memos, Notes on your iPhone etc...

The question is when did you backup on iTunes and when did you delete...Read on...

Let us assume...

Date1 : Backup date

Date2 : Message Deletion Date

Date3 : Current Date / Restoration Date

Given that --> Date1 < Date2 < Date3 

Risk of restoring on Date3: You may lose data between the Date1 & Date 3

Mitigation: Create Multiple backup on iTunes for the same device. (By default iTunes creates incremental backup over the existing backup such that you only will have one backup per device on iTunes)

How to create Multiple Backup for the same device.

1. On iTunes Click on Manage Backups
2. Right Click on a Backup and Click Archive
3. Click Backup Now.
4. You will see a fresh backup will get created See the GIF below

1. Restore from Backup dated Date1 (Archived)
2. Retrieve the message and store it separately
3. Restore from Current Backup Date3

Do you have a backup or any other device under same Apple ID that has those messages?