Welcome to Scamwarners 'frightened'.
I don't think there's any recorded example of any military personnel on an active duty assignment overseas ever falling in love with a stranger on the internet and offering to marry them without ever meeting in person. Doing so would be pretty foolish and men aren't programmed to work like that. Just that alone makes it virtually certain that your sister is dealing with a Nigerian scammer who may be located in Nigeria or the Far East.
A copy of an e-mail from him with the full header may reveal what part of the world his messages are originating from - undoubtedly not Afghanistan. If there were a 1% chance he weren't a scammer, the fact that his undying love is hooked up to a money demand makes it certain it's a scammer. If a man is so desperately in love he wouldn't risk it all over money whereas a scammer believes that he can cash in his love but he never intends to even meet your sister. He probably has a wife and kids already.
The fact is that military personnel are well cared for and don't really have any need for money. Bed, board and travel is paid for by the government and they have health and welfare cover, so the scammer has difficulty making up a reason why they need cash. Usually what they make up is pretty stupid.
Scammers like to use Western Union which they are well able to abuse and pick up funds wherever they are located. If you find you can't stop her sending money, try and persuade her to use the service mentioned here
http://travel.state.gov/travel/tips/emergencies/money/money_1224.htmlThis will ensure it's only collected by the person named and in the country where he claims to be located but beware, because the scammer will react like a vampire coming in to contact with daylight if he knows that he can only collect the money in Afghanistan. Scammers know that they can abuse the standard Western Union system.