If no laws were broken, then there may be no legal case. I say "may be" because I have no idea whether he would get into trouble if she went to his commanding officer about it. The marines might have different rules regarding sexual contact with minors - and you are a minor, even if you are past the age of consent.
Don't count on sitting down and having a reasonable discussion about this with your mother. She's clearly upset and won't be able to deal with this rationally and calmly. The best you can hope for is that she won't be able to press charges, and that eventually this will all blow over. Unfortunately, your relationship with the guy is pretty much shot; you won't be able to keep contact with him without causing a major rift between you and your mother, and she is unlikely to ever accept him as your boyfriend. To her, he will always be the man who took advantage of her little girl.
If she does press charges and it goes to trial, you are honor-bound to tell the truth on the stand. If you did it of your own free will, say so. If you felt pressured in any way, say so. If he didn't know you were only 16, make sure to mention that too - but if he was fully aware of it, you'll need to answer that question truthfully. As a U.S. Marine, he should understand that it would be dishonorable for you to lie under oath, and he should accept that you will testify to the truth, even if it makes him look bad.
One way or another, there will be a lot of healing to do in your family. I imagine you're not too pleased with your sister right now for whatever her role in this was, your mother is probably fairly disgusted with you, and I can't help but think there will be some anger issues to sort out for a while to come.
Just a random thought... you might point out to your mother that if she presses charges, he probably won't be going to Iraq - and really, wouldn't that be doing him a favor in a way? If she wants to punish him for what he did, she could do a lot worse than watch him march off to a war zone where his life will be in constant danger for the next two years or so. [ Xenolan's advice column | Ask Xenolan A Question ]
TheWallflower answered Monday June 4 2007, 10:22 pm: If you weren't forced to, just deny it when the police get involved. The poor marine is going to Iraq. He'll be convicted of rape since you are underage and military trials are far, far, worse than civilian trials. Save his ass and deny it.
jdiiiizle answered Monday June 4 2007, 9:42 pm: Tell her that you chose to do it, and he did not force you. If she does decide to file charges, just don't say anything. What can they do if you don't testify? Not much. [ jdiiiizle's advice column | Ask jdiiiizle A Question ]
Neuronautas answered Monday June 4 2007, 9:27 pm: What you have to do is tell the truth.
If you weren't forced to do it, then the poor guy doesn't deserve to get into any trouble.
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