If you want to fit in some exercise what about a fitness dvd to do at home? Would you be able to find time for that (say 30 minutes max 4 or 5 times a week)? Would you stick to it? If so, I would recommend any of the Rosemary Conley ones. And as 4getmenot says, can you walk instead of taking the car sometimes? It's one of the best exercises there is.
As for diet, again I would recommend Rosemary Conley - buy her magazine, there is always a week's menu plan in it, plus additional exerecises and lots of great information.
I can also recommend Tesco e-diets. You have to pay to join and you can choose what kind of diet you prefer and select any number of preferences, depending on whether you have a family or only yourself to look after. The beauty of e-diets is that you can print off your week's shopping list once you're chosen your menu plan.
Fad diets don't work long term. And I'm sorry to say neither does trying to lose weight for a special event, even if it is very important to you, which understandably your sister's wedding is.
Please don't take this the wrong way but I think you need to change your mind-set here - you 'sound' defeated (and desperate) before you start. You need to want to do this FOR YOURSELF. Otherwise this special occasion will hover like a big black cloud getting ever closer - it will depress you but you cannot stop July coming round.
Work at losing 7 or 14 lbs to begin with, then move onto the next half stone or stone, if you break it down into manageable targets it won't seem so bad.
Above all, try not to panic - you can lose a couple of stone at least, probably even the 3 you want, but it will need a bit of work and planning.
Good luck - let us know how you get on!