Do me and my sister have to join the german army?

Well, today me and my sister got a letter from the german army saying that their data shows that we're nearly 18 and its asking us to come to some place in german and if we don't come we could be fined 500 euros or sentenced to jail for 14 days. My first problem is that im not 18 yet :/, my sister is nearly 18 though. Second problem is that we're in england, so we/she cant go to that place (but we are german citizens). Third problem is that there is a mistake with the letter because both of them say my name, but one was sent in an envelop with my sisters name and says "fraulein" instead of "herr". Im CONFUSED!!!, do they want me or my sister or both of us???? Any ideas of what to do? Thank you.