I understand that many illegal immigrants, who have gotten into this country unlawfully, often at risk to their lives and those of their families, find themselves in a situation where they have to “live a lie” — using fake or stolen social security numbers, forged documents, lying about their immigration status, not paying taxes lest they betray their illegal status, and so on. People who risk their lives to get to the US do not make such a decision lightly, sometimes it is their only choice in life.
Do these people ever sin mortally in so doing? It is true that they have to tell lies to keep from being discovered, and they have to keep telling these lies and are unable to promise that they will stop telling them. Are these lies so grave as to constitute one of the three conditions for mortal sin? They are fully aware of what they are doing, but they do not want to tell these lies — given the choice, they’d rather not, but they do not have this choice, short of leaving the country and returning home. Sometimes they have established new lives here — purchased homes, raised their families, perhaps even having gotten married and so on.
It is very easy to argue that the laws are unjust, that they should be able to get visas to be here legally without fear of being deported. We do need the workers, to do jobs that Americans don’t want to do, and their labor enhances the quality of life of all Americans, and makes our country wealthier. They contribute substantially to our country, and we would have real problems if they weren’t here to do the work (agricultural work, construction, housekeeping, poultry processing, landscaping, etc.). A case can also be made that certain territories (the states of the southwestern US) were taken from Mexico unjustly, and that they are doing no more than violating immigration laws that shouldn’t even exist in the first place, if they have settled in these territories — basically, you could argue, they are in Mexican territory under American occupation. We are not the only country that has occupied foreign territory and made it their own, sometimes with “ethnic cleansing” — Pomerania, Silesia, and Prussia incorporated into Poland, eastern Polish territories incorporated into the USSR, and so on.
Even if the sins are venial (taking into account the fact that they are acting under duress by having to lie, or that the lies do not rise to the level of objectively grave sin), I dislike the idea of someone going to confession and saying, in effect, I repent of all my mortal sins, but there are certain venial sins that I have to keep committing, and I cannot resolve to stop them. But that is easy for me to sit back and say, in that I am a US citizen living a comfortable life, and not having to commit continuous venial sins to keep my life from falling apart, and the lives of those I care about.