Paranoia strikes deep....
You don't know the purpose of the visit (at least you didn't say, and maybe you weren't told either)...And while I'm wary about talking to cops WHEN THERE'S a 3rd person involved, I also wouldn't get all fired up about it. I've not had any really negative dealing with local law enforcement (unlike my dealing with local town government and others(zoning, etc.)).
A few years back (as a teenager), my son "went stupid" on me, and got mixed up in something he shouldn't have. He and I (and one of his buddies) were VERY politely asked to come down to the local Sheriff's Office to talk about it. And so we did. And were treated well, even though it turned out the "kids" had gone WAY above and beyond stupid. However, the cops were more than amiable, and released the kids back TO ME, even though technically they could have been charged with a felony, and tossed in jail. But they understood that mine wasn't REALLY involved, just a tagalong that got caught up in a mess. They gained a HUGE amount of respect from me that day (and, in the end, my son and his friend were never charged, and the whole episode put the "fear of God" into them, he hasn't even gotten a speeding ticket since.), because of the way it was handled.
Now, that doesn't, by any means, imply that I trust ALL cops at ALL times, but they certainly coulda dropped the hammer on him, but they acted reasonably and honestly, and I respect them for that.
If you BIL really screwed-the-pooch, you should not have any bad feelings about it. Being evasive, or lying to the cops is just unwarranted.