Yeah. I had cousins who were into drugs. I was in the "slow lane," too, as a kid. Still am, haven't ever smoked weed and I'm thirty-fourteen.
At any rate, I would suggest a meeting with your SIL--maybe your spouse needs to head up this conversation if it's his/her sister. And agree on certain ground rules. If the rules are broken, that family makes sure the user DEPARTS. Period. Or whatever rules you have set up.
This is something up with which I would not put.
Also, talk with your kids. I knew my cousins were doing pot, because my parents told me. They also told me that pot might not be dangerous, per se, but it was a drug, and it did things to your brain. My cousins, also, were cool enough, but also kind of fuck-ups, so it's not like they were making an appealing argument for pot "enhancing" your life. I got the "just say no" speech, which worked well enough for me. I also pretty clearly understood that if I decided to smoke, my parents in NO WAY would support or condone it. My cousins knew I was square (

) and they knew my dad was the kind of guy who would not stand silent if someone was screwing around with his kids. Not a mean or violent man, just decisive, persuasive, and authoritative. I don't think my cousins would've EVER offered me any pot mostly because they feared and admired my dad, to be honest.
And even if pot is increasingly legal, it's not unabridgedly legal. Distribution to minors is always illegal everywhere.