Cranberry juice truthfully isn't going to do anything at all for passing kidney stones. In fact, I would argue it'll do more harm then good because just like you said - it's full of sugar.
Developing kidney stones is largely dependent on your genetics. Some people are genetically predisposed to developing the while other people are not. Kidney stones form when mineral concentrations (mainly calcium) in your urine change. The main thing that causes kidney stones is not drinking enough water. When you don't drink enough water the minerals bind to other substances in the urine and stick together to form a stone.
Being obese, diets high in red meat, calcium, and salt and dehydration are the main risk factors for kidney stones.
Limit the amount of dairy and red meat in your diet, and watch your sodium intake. Also make sure to drink 5-6L of clean water per day, minimum. Remember that things like coffee are diuretics and don't count towards your water total.
Supplementing with a good overall organ and kidney detox supplement like n2guard from n2bm.com is another great idea.