I know in the first one it is said that the reason the Terminator goes around shooting all those Sarah Connors is because a lot of info was lost. A lot of people say why doesn't skynet send one back a few generations earlier to kill Sarah or john assuming it can find out this info. My idea, and it's just my odd ball way of doing things is, what if Sarah or John are the only 2 it can safely kill because one of her relatives actually worked on something that led to skynet.
It might not be sure how but it knows that anyone before sarah could be a key link to it becoming self aware (ignoring T3 and Salvation). If it killed an uncle, mother, or father for all it knows it would never become sentient. Then again I am in favor of the fact that it only tries to kill humanity because it's first "memory" is probably information about Sarah Connor trying to blow it up at the Cyberdyne labs.