Anonymous wrote:And you can make it very general - "we don't let Larla do sleepovers at homes with guns." That way it's not about them and their guns, it's just that you have this rule and they happen to be in violation of it. I certainly don't let my kid do a playdate/sleepover at a house with a gun.
OP, do this. It makes everything about your general policy on guns-equals-no sleepovers -- tell them that you have the same rule for all your son's friends re: sleepovers.
I do think, as someone else noted, that your SIL is going to then try to explain, excuse and justify like crazy because she seems invested in either keeping up appearances or she's in denial. (It's sad.) So expect some "We keep them locked up, we keep the ammo separate" and so on. That's the time to get a glazed look on your face that says you're not going to listen, and you say, I understand, but the rule's the same. Then change the subject immediately, or find a chore to do instantly, or both....
I would refer to the guns and not the drug use simply because if you bring up the latter she will be back in defensive mode and in explain, excuse mode again, only worse, I'd wager.
It sounds, though, like she and the son could use some company and some distraction from a bad home situation. If you are comfortable with it, meet them halfway between your homes when you can and get the kids together at a park, or come up with other outings -- going to a movie together, going to a kids' play or a kids' craft workshop or the Lego store for a Lego building workshop if the kids are both into that --whatever. In other words, do outings where the kids and the adults can all be focused on the activity; that leaves many fewer openings for SIL to bring up sleepovers again. But do consider seeing them. They sound as if they might possibly be reaching out and lonely, since they go home to a real problem every day. You might do them both good by seeing them, but don't invite the cousin for an overnight--as you say yourself, it sets up a reciprocation expectation.