Your 8-year-old nephew starts teaching your kindergartner dirty words, and your brother doesn't notice (or pretends not to).
What you're tempted to do: Cover your child's ears, and scold your potty-mouthed nephew.
What you should do: Calmly ask the boy to clean up his act. Say, "You're older, and it's not appropriate to use these words in front of Matthew." This will make the child and his dad aware of the problem without offending anyone. When they leave, talk to your child about the naughty words: "We don't use that language in our family. Those words can upset people, and they're inappropriate."