: Content often plays on inside jokes, friendly rivalries between the two couples, or "who knows who better" trivia games.
: Keep deeply personal relationship arguments private. Involving a sibling in a minor couple's spat can cause them to hold a grudge against your partner long after you have forgiven them.
Lighthearted pranks where a sibling tests how well the partner knows their significant other, or teams up with the partner to tease the sibling.
If you're interested in this topic, I can: