I know this is an extreme case but the book I am reading is about occupied France in WWII. A woman has lied about a jewish child being from a dead relative to protect it from the Nazis. Is this type lie even a sin or is it a sin that stops a much larger sin?
I would tend to think that it is not a sin at all. It is protecting innocents and those wrongly prosecuted. It is a different form of fighting.
In general, no, and I prefer to avoid both situational ethics and casuistry. However you do mention a very special case, and it is quite complicated. It is a case of war, but it is a sin of war that one might argue should be repented of. So in the cases you mention, I can’t give you an answer; the Ten Commandments prohibit lying and murder, and it is extremely difficult when someone lies to conceal their identity. Now there is another related case: if a Christian is asked about his own faith, he should I think seek to avoid lying to preserve his life at all costs. Martyrdom, our Lord assures us, and the Apostles demonstrated by example, is not to be feared but is a great honor, a crown for the faithful. Rather in that case, one should pray for the courage to resist the temptation to lie, lest one avoid the opportunity to be glorified in that way.
I see lying in the case I out lined as another means of fighting and resistance. War is horrible and should be repented of but also it is something that we have to do from time to time. I am do like St. Augustine's just war theory. It is a horrible necessity but it is one that has to occur all along.
St. Athanasius had a similiar idea, that military service was honorable. Now, St. Basil the Great, while not disagreeing with St. Athanasius on that point, did disagree with St. Athanasius that returning soldiers should immediately partake of the Eucharist on their return; St. Basil’s view was that they should abstain for three years, if possible, after the conclusion of their service in war, in order to repent of the sins of war. On this point I personally take the view of St. Athanasius, but I think the opinion of St. Basil is important and should be considered. Conversely, I am not familiar with St. Augustine’s doctrine of war, but if it is a theory of “Just War”, I might be inclined to oppose it, on the basis that the causation of all wars is injust, even if one of the parties is justified in waging war.
I would agree with St. Athanasius. Military service is honorable and is not in itself a sin. St. Augustine's theory was that war could only be waged in a just manner if it was in defense of ones country, to protect those weaker who could not protect themselves, and maybe one more reason I can't remember.
War is horrible, sinful things are done in war but you can wage war and fight it in a civilized manner. I see the lying to protect the innocent, as in the example I provided, as part of fighting the war. I don't even see that lie as a sin.