Is it a sin to have sex via Skype with your wife when you live far from each other?
Technically, sex isn't taking place since the two of you are not together. What is taking place is that each is masturbating while watching and talking to the other person. Since this is between a husband and wife, it is not wrong. However, it should not be considered a substitution for sex. "Now concerning the things of which you wrote to me: It is good for a man not to touch a woman. Nevertheless, because of sexual immorality, let each man have his own wife, and let each woman have her own husband. Let the husband render to his wife the affection due her, and likewise also the wife to her husband. The wife does not have authority over her own body, but the husband does. And likewise the husband does not have authority over his own body, but the wife does. Do not deprive one another except with consent for a time, that you may give yourselves to fasting and prayer; and come together again so that Satan does not tempt you because of your lack of self-control" (I Corinthians 7:1-5).
You should realize that anything broadcast through the Internet has the potential of being recorded. Private moments between a husband and wife should be kept private.