I wouldn't worry about it too much. For one, you are having twins. You are going to gain weight! Try to eat foods that are good for you, and don't stress too much. If you aren't dealing with morning sickness, perhaps getting a SLIGHT fill might help keep you in control? Sounds like you could have used one before you got pregnant if you were gaining? I say talk to your OB and your surgeon and see what they have to say.
Remember, the health of your babies is your top concern. You can always lose the weight again later. Stay healthy!