Not sure how to answer this, but I'll try. According to classical Christian teaching, Jesus came into the world for one reason, to redeem the relationship that previously existed between God and man. Through sin of Adam and Eve, since God was perfect, God and man became separated. By Jesus' death, a blood sacrifice was paid to atone for man's sin, if in fact man wish to be re-united with God. We still have a choice, after all, love does not compel another man's actions.
So if aliens had sinned and needed redemption, I suppose then Jesus would have died for their sins as well... at least it makes sense to me.
Einstein's theory of special relativity doesn't work exactly like that, but I understand what you're saying. The best way to characterize special relativity in simple terms is that all of physics works exactly the same for all uniformly moving frames of reference. That's why you can stand in your yard and toss a ball up in the air, and the ball will follow Newton's laws of motion for that frame of reference (i.e., the earth rotating around the earth at a relatively uniform velocity). Then you can get in an airplane that is moving at 350mph, and stand in the aisle and toss the same ball in the air, and it'll also obey Newton's law of motion within that frame of reference (note that it must be a uniform frame of reference, so if the plane experiences turbulence things change a bit).
Now somebody on the earth watching the ball moving inside the plane does not see the ball obeying Newton's law of motion from HIS frame of reference on the earth... so what's changed? The answer to that is the basis of special relativity. It's not too noticeable at slow speeds (everything that we do), but it starts to make a bigger deal at high speeds, such as the flow of electrons within semiconductors such as computer chips.
The story of Jesus is not unbelievable, IMO, rather His actual claim to be the son of God is what is remarkable. Based on the standards historians use, Jesus certainly existed 2000 years ago, was born in Bethlehem to a carpenter and a young teenage girl named Mary. He traveled as an itinerant teacher throughout Palestine during the occupation of Palestine by the Romans, and He was crucified by the Romans to pacify the Jewish religious leaders in Jerusalem. That things are agreed on by historians, regardless of whether you are religious or not.
But what is controversial is what Jesus said. He claimed to be God. According to C.S. Lewis, that made Jesus one of three things: (1) a liar, (2) a lunatic, or (3) exactly what he claimed to be... incarnate God in human form. So we have a choice to make. Who is Jesus to you? We all get to make that decision with our own free will.