You already own the firearms legally in IL, where you are a resident? These firearms are legal in IN?
If you can legally possess them in IN, and they are on a property over which you have control, I don't see why you couldn't keep them there.
If IL were to ban these firearms and IN did not, I don't see how you could get in trouble for owning them. If they were illegal to possess in IL, the firearms are not in IL- they are in IN, which is out of their jurisdiction. They aren't illegal in IN, so IN won't have a law to charge you with.
Of course, I'm not claiming to be a lawyer, so my advice is worth exactly what you paid for it. But I don't see why this would be much different than someone in CA owning CA-banned firearms at a vacation home or relative's home in NV.