Thank you everyone for the comments and advice.
I asked my father if he thought my BIL had all his guns locked up, and he said that he’s sure he does, Since my father is also a “gun guy”, and he’s over to my sister’s house nearly every day, I feel a bit better. However, with the comments here, I feel you all have given me the courage (encouragement!) to ask my sister and BIL directly about the gun situation. I think I"ll bring it up casually, ask about the gun safe.
They both have a TON of junk. They both are collectors of just about everything, and BOTH have been this way even growing up. They do throw out trash, but they collect so much stuff - happy meal toys, clothes, beanie babies, small toys for their kids, etc. It’s not just my sister, it’s the two of them together, so at least they don’t fight too much over the condition of the house. On one or two occasions, my sister did mention to me that she was trying to get things in order and get rid of some stuff, but I was recently in my father’s attic, and I saw that she is now bringing her items back to his house.
Is there such a thing as a “functional” hoarder? Kind of like a “functional” alcoholic? The house is a mess, but the bills get paid, they have full time jobs, the kids are fed, clothed, and bathed. Most of me thinks that my BIL would be responsible enough (and my sister would nag him endlessly) that all his guns would be stored unloaded and locked up. But 20 % of me is worried that he is so disorganized with other things, that why should the guns be any different? He doesn’t take care of big-ticket items outside, like the gas grill or the patio furniture or the kayaks or the (you name it). Just lays where it was left, until the wind or rain ruin them.
I would never call CPS, as they have a very bad reputation in that particular county. Also, one of the parents does the home visits for cases similar to this, and so, I can’t even imaging what would happen if I called. I wouldn’t want anyone to lose their job. However, if I ever find out tangible, beyond a doubt evidence that either of their kids is in danger, I would not hesitate to make some immediate calls to family. Thankfully, there are lots of family members on either side who could step in and take the kids for a while.