sit in on something—
1. attend a meeting, discussion, etc., without taking part in it:
- I was allowed to sit in on the deliberations of the board.
2. take part or collaborate in smth.:
- You weren’t invited to sit in on this deal, but you elected to take a hand.
sit on something—
1. be a member of a committee, a jury, etc.:
- Representatives of the workers should sit on the board of directors.
2. (coll.) delay dealing with smth.:
- She’d “sat” on the article until a deadline had galvanized her into putting words on paper.
3. (coll..) examine or investigate a case, etc.:
- The police are still sitting on the mystery of the stolen jewels.