Unmotivated Teen Summer Couch Potato - HELP!!!!!!!

I will NEVER forget the day my oldest (about age 12) and her youngest brother were bickering. He would have been 8-9 years old. In frustration I told them to knock off the arguing, they were too old for that. Both of them looked at me with stunned expressions and in UNISON said "But we are just having fun"  ::: face palm ::: Mom was not having fun! lol  Sending them out of my hearing solved many of their verbal conflicts (assuming there isn't an unfair advantage such as size of one over the other and no physical fights. Sending both to different areas, and making sure I never gave either one of them what they wanted in the fight tended to stop a lot of it. Knowing they weren't going to win stopped much of it.  My three are all less than 3 and a half years between the youngest and the oldest. 

I did find it helpful that everything had a specific owner. So, play items had an owner who could claim first dibs at any time but sharing was encouraged and they quickly learned if they shared freely their sibling was much more likely to share. Some items that were intended to be family toys like the video games was officially owned by me. I had control over who played when, how turns were distributed and other points of refereeing. If they got into an argument about which game to play mom would choose and her choice would be none of the ones the kids had argued over.  Any opposition resulted in a no play penalty for the next hour or rest of the day.