iOS 12 texts going to the wrong family member

Stop sharing the one account. Set up Family Sharing - Apple instead. See also Add or remove your phone number in Messages or FaceTime - Apple Support and ensure the list of "Reachable at" numbers or email addresses is unique for each user.

tt2

After following the instructions above, you may need to delete the corrupted threads or the threads where all of the conversations have merged. You may have to do it a couple of times before it quits combining the threads.

Thanks. 🙂

tt2

I owe you way more than that, but you are quite welcome. 🙂