No, it’s never permissible to sin.
“Sin” is not the spiritual equivalent to “illegal”. Sin means you have betrayed God in some way.
People migrate without documentation for a variety of reasons. It isn’t unusual to escape central America because a person fears for their life or for the lives of their children, as these areas have been known to be excessively dangerous for quite some time.
A person should never violate the civil authorities since we are suppose to obey them, but this has always been muted when having to obey a higher law (hence why the early Church accepted martyrdom instead of obeying the state and sacrificing to pagan deities and such).
So whether a person sins by illegally crossing into Mexico and the US: this is really a difficult question, but no, not necessarily.