I agree with what everyone else has said!
And I think you ought to give some thought to the thinking that led to your decision so you can recognize the rationalizations and avoid them in the future. For example, why did you steer away from the low calorie meal? What did you let influence your decision (e.g. was your sister ordering something high calorie and you were swayed? Or were there pictures on the menu that sucked you in?)? What made you keep going when you had decided to take half home?
Thinking through what you are going to do and then sticking with that decision is hard, but so important (I know you know that - but doing it and knowing it can be worlds apart)! For example, if you go in knowing what you are going to order (your 420 calorie meal), don't even open the menu - it's only going to lead your mind to trying to convince you to eat something different. And if you are going to take half of something home, ask for a take-out container BEFORE you start eating and take half off your plate before you dig in. Also, enlist the help of the person you are eating with if you know you're going to be tempted. For example, tell your sister what you are going to order and ask her to help you make sure you stick to that decision.
You didn't blow it. You blew one meal. Not one day, not one week, not your entire effort. Learn from it and keep going!