We're big on "context dependent swearing" - basically don't swear in front of people you know don't like it, or people you don't want to risk thinking less of you. Our young teens are pretty good at it. They know it's not okay to swear in front of my parents (especially my mom), but my father-in-law expects about 5 curses per hour.
So I guess my suggestion would be see if the kids can minimize the language when you or your kids are around. Your sibling might get pissed as it could be taken as calling him a bad parent. But 5th/6th graders are old enough to talk to directly without bringing the sibling into it. Then if necessary you can talk to the sibling about their own behavior without bringing up their kids.
So I guess my suggestion would be see if the kids can minimize the language when you or your kids are around. Your sibling might get pissed as it could be taken as calling him a bad parent. But 5th/6th graders are old enough to talk to directly without bringing the sibling into it. Then if necessary you can talk to the sibling about their own behavior without bringing up their kids.