I nursed my son Wolfie throughout the twins' pregnancy. He was not nursing too frequently when I got pregnant (he was about 13-14 months or so) and I figured he would lose interest in it, as I had read oh so many times. Not to happen! He became much more interested in it and started nursing a LOT. I was very thankful for it, actually, because it gave me ample opportunity to lay down with him and I also no longer minded nursing him to sleep at night, because it was easier than the alternatives, once I was the size of a small house.
All in all, I gained 80 pounds, had babies who weighed 8 pounds 11 ounces and 7 pounds 10 ounces, and carried to 40 weeks. I think the quality and quantity of food is vital during a twin pregnancy, but I don't think weight gain is the end-all indicator. My friend just had her twins at 39 weeks, both almost 7 pounds, and she gained about 55 pounds. Her daughter nursed throughout the pregnancy as well and she is now 16 months.
Anyway, once the twins were here, I nursed Wolfie for about a month and then decided to wean him. It was a very positive decision for us, as I needed to let go of the "baby" idea with him and let him become the big brother. I hated having to constantly set limits on him that weren't there before ("waiting his turn," etc.). He became a much different kid, actually, and I know it was the right time for him. Our relationship improved for the better.
He is still very fond of my breasts but does not nurse. Sometimes, he puts a hand on one and just rests it there for comfort.