Map<ID,Opportunity[]> oppsByAccount = New Map<ID,Opportunity[]>();

    for(Opportunity o : [Select AccountID, (Select Pricebookentry.product2.ProductFamily From Opportunity Line Item) From Opportunity Where AccountID IN :THESET]){

       Opportunity[] tmp = oppsByAccount.get(o.AccountID);
       if(tmp == null){
         oppsByAccount.put(o.AccountID,New Opportunity[]{o});
       }else{
           oppsByAccount.get(o.AccountID).add(o);
       }

    }