Can anyone see your text messages?

It is possible yes.

There are a few ways to do this. One method, phone cloning, lets someone intercept incoming messages and send outgoing ones as if their phone were the original. If both phones are near the same broadcast tower, they could also listen in on calls.

To clone a phone, the person would have to make a copy of its SIM card, which stores the phone's identifying information. This requires a SIM reader that can read the card's unique cryptographic key and transfer it to another phone. (Warning: This is super illegal.)

To prevent someone from doing this, make sure that your phone is always directly in your possession, never left around where other people have easy access.