If you must stay with them and you're okay with storing the guns locked in your car, why was it necessary to even bring this up? There's no point in trying to change her mind IMO, as she is clearly irrational. It's going to take more than just your one visit to straighten her out.
And why, BTW, would you want to stay with someone who's so irrational? My bet is you're gonna have other conflicts as well.
I suspect they may be Democrats. Hasbeen
