Did you say sibling conflict is needed?

Yes, siblings need to have conflict, so they can learn ways to move through obstacles. Here are a few things to try;

Talk It Out
Step 1. Calmly empathize
Step 2. Recognize and state the problem
Step 3. Ask them to tell their feelings without hurting each other
Step 4. Remind them of your family rules
Step 5. Ask what they suggest to do about it

Make Light of It - make them laugh
Choice 1. Ask them to fight about it
Choice 2. Switch the subject to something funny
Choice 3. Act as if you're a wrestling announcer or referee
Choice 4. Play a bonding game.
Choice 5. Nap time
Choice 6. Have a fight night with agreeable rules

Try different tactics each time as they mature. These are the times while they are learning what works best for them.