D..
Separate, separate, separate. No matter who instigated it. This way, you don't have to see who started it. You get to be like the Army - if one misbehaves, they all pay the price. Fun activities are dropped like a hot potato and they are both in their separate rooms and you can even make them do a not-so-fun chore once they are out of time-out. (And a LONG time-out for their ages, too.) If they start fighting outside of the home, back home you go, regardless of where you are. (Hard for you, I know.) More stuck in their room time, and another unloved chore.
If you instigate this approach at the very beginning of the summer and follow-through with 100% consistency, and be TOUGH about it 100% of the time, they will start telling each other to behave before M. separates them. Every bit of the fun of picking on each other will be gone if you do this.
I promise it will work if you DO it, and do it every single day until they cave in. They will still test you after you think all is smooth sailing, and then you have to POUNCE again to make them know that the rules are still the rules.
You have the power, Mom. Just Do It.
Dawn