First off, your BIL should have never asked, He should have waited till he was invited. Second, 40 acres is just about the right size for another person to screw your hunting up for you. They would both be looking for a place to hunt if it was me. Hell, my wife hunts with me, her stands are about a mile from mine and she lets me have sex with her!
Hope you work things out without causing a stink, good luck.