Jim is Jack's uncle. But Jack says he is Jim's uncle. Is this possible, without incest or other naughtiness?

By 'uncle', I mean a relationship by blood, not by marriage, adoption, etc.

A solution is in the continuation. You may think of others.

Continuation

John marries Jane. They have a daughter, Jill. Jill marries Julian, and they have a son, Jim. But Jill dies, and Julian marries Jean. Julian and Jean have a daughter, June. After about 18 years, John's wife Jane dies, and John, being a bit of an old goat, marries June - quite properly, as they are not blood relatives. John and June have a son, Jack.

Jim is Jack's uncle, since his half-sister June is Jack's mother. But Jack is also Jim's uncle, as his half-sister Jill is Jim's mother.

I dare say this has actually happened somewhere in Arkansas.