My stepson borrowed my car and had an accident and injured someone

While I was working in Afghanistan, my stepson was in the US on vacation from China and stayed at our house (my wife was in China at the time). He borrowed my SUV (without my knowledge, but his w/mothers permission) and apparently cut-off a fellow on a bicycle that then crashed (though he did not hit my SUV). My insurance (Geico) has covered it, but now they've sent me a letter stating this mans lawyer wants me to sign a form stating that my liability insurance cap is 50k, and I have no additional insurance to tap into (which is true). I'm still working in Afghanistan and have the feeling this person is bleeding my insurance dry and may then sue me. I know nothing of this person, or the nature/extent of his injuries. I don't even know his name. My stepson went back to China, but is a US citizen.What is the extent of my liability? Am I screwed? Currently speaking on the phone isn't possible due to the time zones, but I'll be in the US in two weeks at the number below.